doc: clarify descriptions for tr --squeeze-repeats

doc: clarify descriptions for tr --squeeze-repeats

* doc/coreutils.texi (uniq invocation): Fix the description of
problematic input to say "blank lines" rather than "two or more
blank lines".
(tr invocation): Clarify that -s only works with SET1 when
not translating (when SET2 not specified).  Also explicitly
state in examples where blank lines are deleted.
Also add "deleting" to the menu item.
* src/tr.c (usage): Improve the -s summary to say it always
operates on the last specified SET.