Docs: add comment about empty format context range

doc/api.rst

@@ -210,6 +210,10 @@ template and implement ``parse`` and ``format`` methods::
       // the formatter should parse the 'f' specifier and return an iterator
       // pointing to '}'.
       
+      // Please also note that this character range may be empty, in case of
+      // the "{}" format string, so therefore you should check ctx.begin()
+      // for equality with ctx.end().
+
       // Parse the presentation format and store it in the formatter:
       auto it = ctx.begin(), end = ctx.end();
       if (it != end && (*it == 'f' || *it == 'e')) presentation = *it++;