[bug-gawk] mention how to specify file names containing = in the doc

[Ed Morton]

Documentation enhancement request:

Someone recently asked me how to pass a file name containing an `=` sign 
(e.g. a file named `count=1`) as the input file name argument to awk. 
They had tried this which of course fails:

    awk 'script' count=1

and I glibly told them it was well documented that you should do it as:

    awk 'script' ./count=1

Then I tried to find the documentation to provide a reference for that 
and I couldn't.

The gawk manual in 
https://www.gnu.org/software/gawk/manual/gawk.html#Other-Arguments tells 
us that `count=1` on it's own would be taken  as a variable assignment 
and then 
https://www.gnu.org/software/gawk/manual/gawk.html#Assignment-Options 
confirms that fact but I couldn't find anywhere that tells us how to 
solve the issue by just sticking `./` in front of the file name.

Maybe we could get that stated somewhere in the manual?

    Ed.