Doc: improve description of regexp character classes.

Define the meanings of the POSIX-spec character classes in line,
rather than referring to the ctype(3) man page.  That man page
doesn't even exist on many modern systems, and if it does exist
it probably says the wrong things about non-ASCII characters.
Also document our non-POSIX-spec "ascii" character class.

Also, point out here that this behavior is controlled by collation or
LC_CTYPE, since the existing text explaining that is pretty far away.

Per gripe from Geert Lobbestael.  Given the lack of prior complaints,
I'm not excited about back-patching this.

Discussion: https://postgr.es/m/155837022049.1359.2948065118562813468@wrigleys.postgresql.org

doc/src/sgml/func.sgml

@@ -5104,18 +5104,37 @@ SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
    <para>
     Within a bracket expression, the name of a character class
     enclosed in <literal>[:</literal> and <literal>:]</literal> stands
-    for the list of all characters belonging to that class.  Standard
-    character class names are: <literal>alnum</literal>,
-    <literal>alpha</literal>, <literal>blank</literal>,
-    <literal>cntrl</literal>, <literal>digit</literal>,
-    <literal>graph</literal>, <literal>lower</literal>,
-    <literal>print</literal>, <literal>punct</literal>,
-    <literal>space</literal>, <literal>upper</literal>,
-    <literal>xdigit</literal>.  These stand for the character classes
-    defined in
-    <citerefentry><refentrytitle>ctype</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
-    A locale can provide others.  A character class cannot be used as
-    an endpoint of a range.
+    for the list of all characters belonging to that class.  A character
+    class cannot be used as an endpoint of a range.
+    The <acronym>POSIX</acronym> standard defines these character class
+    names:
+    <literal>alnum</literal> (letters and numeric digits),
+    <literal>alpha</literal> (letters),
+    <literal>blank</literal> (space and tab),
+    <literal>cntrl</literal> (control characters),
+    <literal>digit</literal> (numeric digits),
+    <literal>graph</literal> (printable characters except space),
+    <literal>lower</literal> (lower-case letters),
+    <literal>print</literal> (printable characters including space),
+    <literal>punct</literal> (punctuation),
+    <literal>space</literal> (any white space),
+    <literal>upper</literal> (upper-case letters),
+    and <literal>xdigit</literal> (hexadecimal digits).
+    The behavior of these standard character classes is generally
+    consistent across platforms for characters in the 7-bit ASCII set.
+    Whether a given non-ASCII character is considered to belong to one
+    of these classes depends on the <firstterm>collation</firstterm>
+    that is used for the regular-expression function or operator
+    (see <xref linkend="collation"/>), or by default on the
+    database's <envar>LC_CTYPE</envar> locale setting (see
+    <xref linkend="locale"/>).  The classification of non-ASCII
+    characters can vary across platforms even in similarly-named
+    locales.  (But the <literal>C</literal> locale never considers any
+    non-ASCII characters to belong to any of these classes.)
+    In addition to these standard character
+    classes, <productname>PostgreSQL</productname> defines
+    the <literal>ascii</literal> character class, which contains exactly
+    the 7-bit ASCII set.
    </para>
 
    <para>
@@ -5126,8 +5145,7 @@ SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
     and end of a word respectively.  A word is defined as a sequence
     of word characters that is neither preceded nor followed by word
     characters.  A word character is an <literal>alnum</literal> character (as
-    defined by
-    <citerefentry><refentrytitle>ctype</refentrytitle><manvolnum>3</manvolnum></citerefentry>)
+    defined by the <acronym>POSIX</acronym> character class described above)
     or an underscore.  This is an extension, compatible with but not
     specified by <acronym>POSIX</acronym> 1003.2, and should be used with
     caution in software intended to be portable to other systems.