docs: extend the URL API descriptions

Closes #10701

docs/libcurl/curl_url_get.3

@@ -41,6 +41,10 @@ The \fIwhat\fP argument should be the particular part to extract (see list
 below) and \fIpart\fP points to a 'char *' to get updated to point to a newly
 allocated string with the contents.
 
+The URL API has no particular maximum length for URL fiends. In the real
+world, excessively long field in URLs will cause problems even if this API
+accepts them. This function can return very large ones.
+
 The \fIflags\fP argument is a bitmask with individual features.
 
 The returned part pointer must be freed with \fIcurl_free(3)\fP after use.
@@ -91,26 +95,38 @@ anything outside the ASCII range.
 .IP CURLUPART_URL
 When asked to return the full URL, \fIcurl_url_get(3)\fP will return a
 normalized and possibly cleaned up version of what was previously parsed.
+
+We advice using the \fICURLU_PUNYCODE\fP option to get the URL as "normalized"
+as possible since IDN allows host names to be written in many different ways
+that still end up the same punycode version.
 .IP CURLUPART_SCHEME
 Scheme cannot be URL decoded on get.
 .IP CURLUPART_USER
 .IP CURLUPART_PASSWORD
 .IP CURLUPART_OPTIONS
+The options field is an optional field that might follow the password in the
+userinfo part. It is only recognized/used when parsing URLs for the following
+schemes: pop3, smtp and imap. The URL API still allows users to set and get
+this field independently of scheme when not parsing full URLs.
 .IP CURLUPART_HOST
 The host name. If it is an IPv6 numeric address, the zone id will not be part
 of it but is provided separately in \fICURLUPART_ZONEID\fP. IPv6 numerical
 addresses are returned within brackets ([]).
+
+IPv6 names are normalized when set, which should make them as short as
+possible while maintaining correct syntax.
 .IP CURLUPART_ZONEID
 If the host name is a numeric IPv6 address, this field might also be set.
 .IP CURLUPART_PORT
-Port cannot be URL decoded on get.
+A port cannot be URL decoded on get. This number is returned in a string just
+like all other parts. That string is guaranteed to hold a valid port number in
+ASCII using base 10.
 .IP CURLUPART_PATH
-\fIpart\fP will be '/' even if no path is supplied in the URL.
+The \fIpart\fP will be '/' even if no path is supplied in the URL. A URL path
+always starts with a slash.
 .IP CURLUPART_QUERY
-The initial question mark that denotes the beginning of the query part is
+The initial question mark that denotes the beginning of the query part is a
-a delimiter only.
+delimiter only.  It is not part of the query contents.
-It is not part of the query contents.
-
 
 A not-present query will lead \fIpart\fP to be set to NULL.
 A zero-length query will lead \fIpart\fP to be set to a zero-length string.
@@ -118,6 +134,8 @@ A zero-length query will lead \fIpart\fP to be set to a zero-length string.
 The query part will also get pluses converted to space when asked to URL
 decode on get with the CURLU_URLDECODE bit.
 .IP CURLUPART_FRAGMENT
+The initial hash sign that denotes the beginning of the fragment is a
+delimiter only. It is not part of the fragment contents.
 .SH EXAMPLE
 .nf
   CURLUcode rc;

docs/libcurl/curl_url_set.3

@@ -54,6 +54,10 @@ does not know about, the \fBCURLU_NON_SUPPORT_SCHEME\fP flags bit must be
 set. Otherwise, this function returns \fICURLUE_UNSUPPORTED_SCHEME\fP on URL
 schemes it does not recognize.
 
+This function call has no particular maximum length for any provided input
+string. In the real world, excessively long field in URLs will cause problems
+even if this API accepts them.
+
 The \fIflags\fP argument is a bitmask with independent features.
 .SH PARTS
 .IP CURLUPART_URL
@@ -66,16 +70,25 @@ will be replaced with the information of the newly set URL.
 Pass a pointer to a null-terminated string to the \fIurl\fP parameter. The
 string must point to a correctly formatted "RFC 3986+" URL or be a NULL
 pointer.
+
+Unless \fICURLU_NO_AUTHORITY\fP is set, a blank host name is not allowed in
+the URL.
 .IP CURLUPART_SCHEME
 Scheme cannot be URL decoded on set. libcurl only accepts setting schemes up
 to 40 bytes long.
 .IP CURLUPART_USER
 .IP CURLUPART_PASSWORD
 .IP CURLUPART_OPTIONS
+The options field is an optional field that might follow the password in the
+userinfo part. It is only recognized/used when parsing URLs for the following
+schemes: pop3, smtp and imap. This function however allows users to
+independently set this field at will.
 .IP CURLUPART_HOST
 The host name. If it is IDNA the string must then be encoded as your locale
 says or UTF-8 (when WinIDN is used). If it is a bracketed IPv6 numeric address
 it may contain a zone id (or you can use CURLUPART_ZONEID).
+
+Unless \fICURLU_NO_AUTHORITY\fP is set, a blank host name is not allowed to set.
 .IP CURLUPART_ZONEID
 If the host name is a numeric IPv6 address, this field can also be set.
 .IP CURLUPART_PORT