doc tweak re backslashes in bracket expressions

[Ed Morton]

Just a small tweak suggestion for the gawk documentation regarding 
backslashes inside bracket expressions.
https://www.gnu.org/software/gawk/manual/html_node/Bracket-Expressions.html 
currently says (**emphasis mine**):
> The treatment of ‘\’ in bracket expressions is compatible with other 
> awk implementations **and is also mandated by POSIX**. 
but POSIX, at least this 2024 incarnation of the spec, seems pretty 
clear (see references below*) that a backslash inside a bracket 
expression is not an escape character so per POSIX these would be 
compliant behavior:
> $ printf 'a\\d\n' | grep -E '[\]'
> a\d
> $ printf 'a\\d\n' | sed -En '/[\]/p'
> a\d
while these would not:

> $ printf 'a\\d\n' | awk '/[\]/'
> awk: cmd. line:1: /[\]/
> awk: cmd. line:1:  ^ unterminated regexp
> $ printf 'a\\d\n' | awk --posix '/[\]/'
> awk: cmd. line:1: /[\]/
> awk: cmd. line:1:  ^ unterminated regexp
so maybe either remove that "and is also mandated by POSIX" statement or 
provide a reference to where that behavior IS mandated by POSIX to clear 
up any confusion.
    Ed.

*From the current, 2024, POSIX regexp spec, 
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html 
(**emphasis mine**):
> > [9.1 Regular Expression 
> Definitions](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html#tag_09_01) 
>
> > ...
> > escape sequence
> >
> > The escape character followed by any single character, which is
> > thereby "escaped". The escape character is a \<backslash\> that is
> > **neither in a bracket expression** nor itself escaped.
which tells us that a backslash within a bracket expression is not an 
escape character, and this:
> > [9.3.5 RE Bracket 
> Expression](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html#tag_09_03_05)
> >
> > ... When the bracket
> > expression appears within an ERE, the special characters ... and 
> '```\```' (...
> > and \<backslash\>, respectively) shall **lose their special meaning 
> within
> > the bracket expression**
which reiterates that a backslash within a bracket expression has no 
special meaning, and there's nothing I can see in [the POSIX awk 
spec](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/awk.html) 
to override the above definitions.