Fix ic textobject mapping and document it

[atomictom]

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Currently the mapping for the ic (inline comment) mapping has no space in between the omap and g:tcomment_textobject_inlinecomment parts meaning that the whole expression would look like:

exec 'omapic <Plug>TComment_ic' rather than exec 'omap ic <Plug>TComment_ic'.

I also mentioned this in https://github.com/tomtom/tcomment_vim/issues/255#issuecomment-569766836 and partially addresses https://github.com/tomtom/tcomment_vim/issues/237.

I've also added some brief docs that acknowledge the existence of this functionality as well as how to change or disable the mapping.

This also addresses #255 and #188. It also partially addresses #237 (by making it clear that the functionality exists, even if it doesn't quite do what's being asked for there).

[tomtom]

1. I cannot currently look into this. I'd be surprised if the period solved a problem -- please see :help :exec.
2. The docs are generated from comments in the source. Please add the docs there.

Thanks.

[atomictom]

1. I cannot currently look into this. I'd be surprised if the period solved a problem -- please see :help :exec.

No rush! It's the added space that should fix the issue. The periods are just to be consistent in style with the code above it (which also use them).

2. The docs are generated from comments in the source. Please add the docs there.

As far as I can tell, this is only true for the commands and maybe some of the functions, but I can't see where this is done for individual options.

But, maybe I'm missing something so let me know if I am and I can fix it up!