Removed confusing paragraph from the docs.

This snippet wasn't particularly helpful as `add_error()` is described
in greater details in the following section.

Thanks Claude Paroz for the report.
Added "Features removed in 1.8" section to release notes.
Removed redundant link in 1.7 release notes.
Added a dummy class for HTMLParserError; refs #23763.
Fixed #9893 -- Filename could exceed max_length on a collision

Passing max_length argument into the Storage's save() and
get_available_name() methods.
Rewriting get_available_name() so filenames do not exceed max_length
when there is a collision.
Writing tests for the case.