[docs][syscalls] Add TODOs about documenting system call rights

ZX-2399 #comment

Test: no functional change
Change-Id: I1f285c677e2444fcf1d720d7ecbd9f490c303962

[docs][syscalls] Clarify ZX_ERR_NO_MEMORY as eventually going away

Not touched:
vmo_op_range.md
vmo_set_size.md
vmo_write.md
The current text is ok for these.

ZX-2303 #comment patch

Tested: Just doc changes, read before/after.

Change-Id: I2c6e1039e2b205e9cc377b9888755fa7ac676c78

[docs] Fix thread state documentation.

Addresses some problems with the thread_read/write_state API documentation.

Change-Id: Ie0b76c2eb332d67b81a3843760913423d061f42b

[debugger] Clarify thread state API

Firms up the contract for zx_read|write_thread_state and makes the state more
explicit for each platform.

The general register struct has been made for the current platform which
simplifies much fo the code.

The sizes are converted to size_t for consistency with other object calls.

Forwarding #defines and typedefs are included for downstream users until they
can be updated.

This will likely need to be revised somewhat again when we add more thread
states, which I plan to do in a followup.

Change-Id: I57e8e9979d073c8efdfaeb2683f9f83494d19abf

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[docs][errors] Rename all the errors in docs/ to MX_OK and MX_ERR_*

Change-Id: I62451456a83145760c6454a619d4c699ce8e9017

[docs][syscalls] Fix all the manpages

Change-Id: Ie9ba25060f3331de8d9c178caf6332fb48fc80f5

[kernel][syscalls] Add docs for thread_r/w_state

Also recognize ERR_NOT_SUPPORTED,ERR_UNAVAILABLE in debugger
test as they're not fatal errors.

Change-Id: I0e35b726619829a2861aa07b3c722aee7b6f5bad