Document some 'mark'-related commands.

I can't really figure out what 'yank' does. It seems to paste the text deleted when running 'delete_to_mark' but that doesn't seem to be coherent with its name...
Correct small typo.
Rephrase and simplify mark-related commands.

Now covering multiple selections. Harder to explain than to see.
Fix another typo.
Merge pull request #71 from angelsanz/patch-1

Document some 'mark'-related commands. (+ fix small typo)