Rephrase wording on giterr_last documentation

Author: RandomSort

docs/error-handling.md

@@ -117,8 +117,11 @@ The public error API
   the last error.  This may return NULL if no error has occurred.
   Otherwise this should return a `git_error` object indicating the class
   of error and the error message that was generated by the library.
-  Do not use this method unless you received an error, it will 
-  lead to undefined behaviour.
+  Do not use this function unless the prior call to a libgit2 API
+  returned an error, as it can give misleading results.
+  libgit2's error strings are not cleared aggressively, 
+  and this function may return an error string that reflects a prior error,
+  possibly even reflecting internal state.
 
   The last error is stored in thread-local storage when libgit2 is
   compiled with thread support, so you do not have to worry about another