Many comments added and docstrings fixed.

Many comments added and docstrings fixed.
(tags-table-list): Elt of nil is not special.
(tags-expand-table-name): Value of nil is not special.
(tags-next-table): Removed arg RESET; no caller used it.
(visit-tags-table-buffer): Don't need to do tags-expand-table-name in or form.
When table is invalid, only set tags-file-name to nil globally if its
global value contained the losing table file name.
(find-tag-tag): Return a string, not a list.
(find-tag-noselect, find-tag, find-tag-other-window, find-tag-other-frame):
Changed callers.
(etags-recognize-tags-table): Call etags-verify-tags-table, rather than
duplicating its functionality.
(visit-tags-table-buffer): When CONT is 'same, set it to nil after the
cond.  We want the normal list frobbing to take place in this case.
(find-tag-other-window): Save and restore window-point around call to
find-tag-noselect.