Help: Fix documentation of escape sequences in a regex

Documentation added by commit 4b35dab891 (Help: Document how escape
sequences work in a regex, 2018-07-18) is only correct for backslashes
inside `[]` groups.  The regex engine does interpret `\` escapes
elsewhere.  Fix the docs.

Inspired-by: R2RT <artur.ryt@gmail.com>
Fixes: #18428
1 file changed