regcomp.c: Expand diagnostic

Perl 5.16 added a deprecation message for an unescaped left brace
following \b and \B in regular expression patterns.  It suggests
escaping the brace by preceding it with a backslash.  This doesn't work
if the pattern delimiters are {}.  Instead, one can escape the brace
by enclosing it within square brackets.  In recent Perls, this compiles
identically to the backslashed version.

This comit expands the message to suggest both options.