giterr_set_str: remove GITERR_OS documentation

The `giterr_set_str` does not actually honor `GITERR_OS`.  Remove
the documentation that claims that we do.

Author: Edward Thomson

include/git2/errors.h

@@ -126,11 +126,6 @@ GIT_EXTERN(void) giterr_clear(void);
  * This error message is stored in thread-local storage and only applies
  * to the particular thread that this libgit2 call is made from.
  *
- * NOTE: Passing the `error_class` as GITERR_OS has a special behavior: we
- * attempt to append the system default error message for the last OS error
- * that occurred and then clear the last error.  The specific implementation
- * of looking up and clearing this last OS error will vary by platform.
- *
  * @param error_class One of the `git_error_t` enum above describing the
  *                    general subsystem that is responsible for the error.
  * @param string The formatted error message to keep