perldiag, perlre: Clarify capture group name rules

Author: khwilliamson

pod/perldiag.pod

@@ -2867,8 +2867,9 @@ has since been undefined.
 S<<-- HERE> in m/%s/
 
 (F) Group names must follow the rules for perl identifiers, meaning
-they must start with a non-digit word character.  A common cause of
-this error is using (?&0) instead of (?0).  See L<perlre>.
+that ASCII-range ones must start with a non-digit word character.  A
+common cause of this error is using (?&0) instead of (?0).  See
+L<perlre> and L<perldata/Identifier parsing>.
 
 =item ()-group starts with a count
 

pod/perlre.pod

@@ -1198,7 +1198,9 @@ You can dispense with numbers altogether and create named capture groups.
 The notation is C<(?E<lt>I<name>E<gt>...)> to declare and C<\g{I<name>}> to
 reference.  (To be compatible with .Net regular expressions, C<\g{I<name>}> may
 also be written as C<\k{I<name>}>, C<\kE<lt>I<name>E<gt>> or C<\k'I<name>'>.)
-I<name> must not begin with a number, nor contain hyphens.
+I<name> must follow the rules for perl identifiers
+(L<perldata/Identifier parsing>) which means, for example, that they
+can't begin with a number, nor contain hyphens.
 When different groups within the same pattern have the same name, any reference
 to that name assumes the leftmost defined group.  Named groups count in
 absolute and relative numbering, and so can also be referred to by those