doc: use semicolon instead of period in option descriptions

doc: use semicolon instead of period in option descriptions

Also do not end option descriptions with a period, properly indent
continuation lines, and make some tiny clarifications.

* src/du.c (usage): Lowercase after semicolon.
* src/ls.c (usage): Semicolons instead of periods, small rephrasing
and two hyphens for clarity, proper indentation.
* src/mktemp.c (usage): Semicolons and lowercase.
* src/od.c (usage): Semicolons.
* src/ptx.c (usage): Use the standard phrase, clarify default option.
* src/setuidgid.c (usage): Properly indent continuation line.
* src/split.c (usage): Semicolons, lowercase, no final period.
* src/stat.c (usage): Semicolons, lowercase.
* src/tail.c (usage): Proper indentation, one shorter rephrasing,
semicolons, no final periods.
* src/timeout.c (usage): Properly indent, semicolons, no final periods.
Fixes http://bugs.gnu.org/14976