modified ZSTD_CCtx_reset()

modified ZSTD_CCtx_reset()

which now accepts an enum,
to distinguish between resetting the session, or the parameters (or both).

removed ZSTD_CCtx_resetParameters(), which is redundant.

start replacing invocation of ZSTD_CCtx_reset*() functions

Updated advanced API documentation

trimmed down amount of API staged in RC,
in particular, all functions related to ZSTD_CCtxParams()
seem too advanced.