Minor documentation change suggestion

[Jakub Martisko]

Hi,

according to [1], the escaped quote should be not used in
regexps. This is partialy hinted in [2], however the wording there is a
bit unclear in my opinion. The "necessary for string constants only"
part should be a bit more strict (e.g. "should be used for string constants
only"). My reasoning for this is that when I use something which is not
neceassary, it should not lead to a warning.

Thanks,
Jakub


[1]:https://lists.gnu.org/archive/html/bug-gawk/2019-08/msg00051.html
[2]: 
https://www.gnu.org/software/gawk/manual/html_node/Escape-Sequences.html#Escape-Sequences