rfc36: example of a "fully commented out" directive

[wihobbs]

Not sure if this belongs in RFC 36 or the manual pages, but @bryujin asked us to document how to fully comment out a submission directive (i.e. not just part of one.) Example:

#!/bin/sh
# flux: --job-name=hello
# flux: -N 2 
# flux: # --amd-gpumode=CPX # comment this out to specify the gpumode
# flux: --env=-* # this is a comment _within_ a directive
#

...

[grondo]

It should probably go in both places! One nit, the comment says "command this out to specify the gpumode" but probably means "uncomment this line to specify the gpumode".

There was also a complaint that this is unintuitive, which is fair. However, it may be more intuitive if we can tweak the docs to ensure users understand that the text between the first and last valid sentinel form a document within the comments that comprises the batch directives. This may make it more clear that you can completely comment out an entire line to make it into a no-op, or part of a line to comment out part of the directive.

Alternately, if we can devise some way to make it intuitive, we could try to do that.