Provide examples of tor_api_get_provider_version() output

At the same time, sternly warn any person thinking about relying on
any particular format too strictly.  If you do this, and your
program breaks, it is your bug, not mine.

src/feature/api/tor_api.h

@@ -65,6 +65,12 @@ void tor_main_configuration_free(tor_main_configuration_t *cfg);
  * libtorrunner, not the version of Tor that it's invoking for you.
  *
  * Added in Tor 0.3.5.1-alpha.
+ *
+ * Example return values include "tor 0.3.5.1-alpha" when linked directly
+ * against tor, and "libtorrunner 0.3.5.1-alpha" when linked against
+ * libtorrunner while it is invoking an arbitrary version of Tor.  HOWEVER,
+ * the user MUST NOT depend on any particular format or contents of this
+ * string: there may be other things that implement Tor in the future.
  **/
 const char *tor_api_get_provider_version(void);