docs: Revise notes for example std_global_allocator.rs

[polarathene]

I noticed a recent commit hoisted the two type comments from their contextual proximity to relevant code, while the START_ARENA line seems to have no relevance separating the two comment sections. Grouped them together as bullet point notes.

The two type focused comments are more explicit about communicating that early now, followed by the description for the choice (it's not really communicated why spin::Mutex is sensible to the user, but I suppose that's better context than none). Was vaguely touched on here, I could add a link to that for reference if considered helpful?

It's not clear what the Miri comment refers to by "this", other than the relation to the global allocator attribute. Nor what "if you can't have that" is referring to (the violation being potentially valid, or a false-positive from Miri?). Additionally corrected a typo + corrected the file name that was changed since the comment was written.

The shorter README equivalent uses ARENA instead of START_ARENA, was START_ meant to communicate any additional context?

• This isn't an area of expertise for me, so while it might not matter to someone more familiar, for a newbie however there might be some benefit in adding context.
• Your response in this PR mentions multiple arena's, so I guess the intent of START_ is an initial arena.

[SFBdragon]

Hey Brennan, thanks for the contribution! Got a packed week but I'll try get around to this within a few days ^_^

[SFBdragon]

Going to merge this as-is, it's an improvement. Going to make some additional changes, I'll write docs with regards to your suggestions as well (as well as some other notes from #32 ).