Major reworking of the pepper section

[chrisdlangton]

This will remove all mentions of storing a pepper (application secret) in a config file.
The alternatives described in the Disadvantages section were covering the same ground, so i reworked these to be in their own Alternatives section with more context on how they differ from the common pepper method.
Also removed references to ambiguous and irrelevant words like traditional and use more appropriate terms that demonstrate a clear message.
Also introduces the concept of CSPRNG to pepper generation, which is critical for security of session which share the similar characteristics to a pepper.
Updated the concatenation to a prefix approach which mitigates known attacks against peppering and avoid some implementation bugs such as truncation.

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[jmanico]

omg this is so fabulous and my only regret is not reviewing this sooner