[SH] Document constraints, remove unused Sr0 constraint

[Oleg Endo]

Hi,

The attached patch updates the top comment block in constraints.md and
removes the unused Sr0 constraint.
Tested with 'make all-gcc'.

OK?

Cheers,
Oleg

ChangeLog:

	* config/sh/constraints.md: Update list of constraints in 
	comment block.
	(Sr0): Remove unused memory constraint.
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