[documentation] Exhausting the docs

[glcraft]

I think it's missing a lot of interesting information in the documentation.

There should be a description for each operator, or at least the one that may not be evident (of, as, in, to, permute, choose...).

There is also no documentation of the functions available in fend, meaning that we don't know what function is available (I misread the "Functions and Constants" part of the docs), nor the utility/description of them.

[mahtar]

I also noticed that. For example I found @plain_number in Changelog but it's not mentioned anywhere in the main manual text.