mode-switching: Tweak the macro/hook documentation

I found the documentation for the mode-switching macros/hooks
a bit hard to follow at first.  This patch tries to add the
information that I think would have made it easier to understand.

Of course, documentation preferences are personal, and so I could
be changing something that others understood to something that
seems impenetrable.

Some notes on specific changes:

- "in an optimizing compilation" didn't seem accurate; the pass
  is run even at -O0, and often needs to be for correctness.

- "at run time" meant when the compiler was run, rather than when
  the compiled code was run.

- Removing the list of optional macros isn't a clarification,
  but it means that upcoming patches don't create an absurdly
  long list.

- I don't really understand the purpose of TARGET_MODE_PRIORITY,
  so I mostly left that alone.

gcc/
	* target.def: Tweak documentation of mode-switching hooks.
	* doc/tm.texi.in (OPTIMIZE_MODE_SWITCHING): Tweak documentation.
	(NUM_MODES_FOR_MODE_SWITCHING): Likewise.
	* doc/tm.texi: Regenerate.

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