Document use of isequal for comparisons

[knuesel]

This documents that the join functions use isequal for key comparisons. It also adds a note that joining on floating point values is not recommended.

I also added the "on is a required argument" sentence to all the relevant docstrings (previously it was only included in the innerjoin and outerjoin docstrings).

The isequal sentence in the matchmissing bullet point seemed redundant so I removed it, but please check, maybe I misunderstood its meaning.

[bkamins]

Looks good. Let us wait for @nalimilan to comment on the language.
I left one small other improvement that we could also make (on can be multiple columns).

[knuesel]

I've reworded the on paragraph as per the discussion above. The text is now generic from the start regarding the number of columns.

[bkamins]

Thank you!