Sort functions in doc index frames

The list of functions per module in the left-hand frame in the HTML docs (e.g., http://www.erlang.org/doc/man/string.html) have been sorted by their order within the associated XML doc file, which is not always alphabetically. This makes quickly finding a function within a module's function index list slower than it would be if sorted, as a full visual scan of the functions needs to be done to attempt to locate any specific function. Or, if the viewer doesn't realize that the list is not sorted, possibly missing a function that does appear in the list.

In some cases the reason for the misordering is that related functions are grouped together in the XML doc (e.g., string:chr/2 and string:rchr/2), as that makes sense when reading the body of the documentation. In other cases, there's no clear reason for the misordering.

Regardless, an index list of functions, such as this, which is sorted for most modules, but not always, is confusing. This change sorts the function index list per module alphabetically, while leaving alone the body of the page. In cases where module callbacks are included in the documentation (e.g., http://www.erlang.org/doc/man/application.html), those functions (e.g., Module:start/2) still remain at the bottom of the index list.