incomplete doc strings for add-text-properties

[Francesco Potorti`]

All the functions in textprop.c that use START END OBJECT parameters
(like for example add-text-properties) do not document what are START
and END.  The same for the elisp manual.

I suppose that START and END are buffer positions or markers if OBJECT
is a buffer, and 0-based indices if buffer is a string, where END set to
nil means the end of the string.

This information should be included in every doc string of those
functions, and in the elisp manual.

Also, I noticed that the substring function does not mention that the
index into the string is 0-based.

I can do that, if it is okay.