Feature/document and streamline rcc interface

[zheylmun]

Added documentation based on review of RCC section of reference manual
Simplified existence tokens and ensure that they can't be created outside of the HAL.

Interface Question:
From reading the reference manual, it's not clear to me that applying a clock configuration "freezes" it.

• If the RCC config is mutable, then maybe the freeze terminology and statement that the existence of a Clocks struct indicates that the clocks cannot be further modified should be reconsidered?

• If the RCC config is fixed somehow once it's applied, maybe it might make sense to remove the clock_src function from the Config implementation, as you only get a config by specifying a src to begin with.