md-files: Add a note about escaped quotes in braced strings in md files

While looking into PR 33532, It was noted that \" would be treated
still as " for braced strings in the md file. I think that is still
the correct thing to do. So let's just a note to the documentation
on this behavior and NOT change read-md.cc (read_braced_string).
Since this behavior has been there for the last 23 years and only
one person ran into this behavior and helped with the conversion
from using quoted strings to braced strings; that is you just need
to remove the quote around the brace rather than change all of the
code.

Build the documentation to make sure it looks correct.

gcc/ChangeLog:

	* doc/rtl.texi: Add a note about quotes in braced strings.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>