More distinctions between asm blocks and assembly code

[joshtriplett]

I reviewed the entire chapter, and added some more disambiguations in cases of possible ambiguity between the entire asm! block (including label blocks) and the assembly code within it.

The second commit, which I've kept separate, attempts to clarify an interaction between noreturn and label blocks; however, for that commit I could use some confirmation on accuracy. The phrasing as previously written said that "A noreturn asm block behaves just like a function which doesn't return", but that isn't accurate in the presence of label blocks, since the asm block can return, from the label blocks. I changed it to apply to "A noreturn asm block with no label blocks". However, this assumes there are not more semantic connotations of noreturn here that do apply to asm blocks with label blocks.