Update docs/user/advanced.rst

Rephrasing a line in the docs, add explicite mention of "client side certificate" (for indexing and clarity)
shorten filename by os.path.basename in guess_filename(obj)
Merge pull request #1152 from ekatsah/patch-1

Update docs/user/advanced.rst
Merge pull request #1153 from cbare/master

use only base filename in multipart file upload