Added brackets around the expansion of SSTRING() just in case.
Changed the name of insert_sstring()'s argument to clarify it's purpose.
Fixed a typo.
Clarified the documentation of find_str_in_sstring().
Changed the return value when sub_str isn't found in find_str_in_sstring().

Reason for that change is that it has overlapped with the value for bad arguments, which could easily become confusing.
Merged an if check at the start of find_str_in_sstring().
Shortened a line to be under the limit.
Added checks for num == 0 and start >= length and documented them.
Removed TODO list.

Shouldn't be necessary anymore as all TODOs are finished and the library should be finished soon.