improve document of loader functions more noticeable

[alexwwang]

Solve the problem mentioned in #20

[pep8speaks]

Hello @alexwwang! Thanks for submitting the PR.

• In the file flask_jwt_extended/jwt_manager.py, following are the PEP8 issues :

Line 309:91: E501 line too long (91 > 90 characters)

[coveralls]

Coverage remained the same at 100.0% when pulling 539ee81 on alexwwang:master into 3d598d4 on vimalloc:master.