Explicit tuning is not well documented

[martinling]

There is no helpful documentation to point to for explicit tuning.

The hackrf_set_freq_explicit() function is not documented.

The help text for hackrf_transfer lists the options, but provides no clues on how to use them.

Using explicit tuning requires understanding the HackRF architecture, which is also not well documented (#1011).

(This came up when answering a question on #1001)