doc: more semicolons instead of periods in option descriptions

doc: more semicolons instead of periods in option descriptions

Also slightly rephrase some descriptions for extra clarity, and
add more consistent indentation.

* src/df.c (usage): Semicolon, no final period.
* src/du.c (usage): Likewise, plus indentation and clarifying words.
* src/ls.c (usage): Semicolon, rephrasings, added parentheses for
clarity, indentation.
* src/rm.c (usage): Semicolons.
* src/tail.c (usage): Adjust -f description to prefer explanatory
language instead of option syntax.