Improve sig snippet to display alias statement

[sae220]

Before @ganicke 's PR (#10078), alias statements have same reference (or help doc) as original statements have.

Thanks to his PR, _removeAtStatement (alias for removeAt) has an original page, but it is not altogether good, I guess.
For blocks users, removeAt and _removeAtStatement looks different statements, but they are essentially the same.
And of course, JavaScript/Python users shouldn't use _removeAtStatement.

So, I suggest to improve sig snippet to display alias statement.

And anyway _popStatement, _shiftStatement and _unshiftStatement (as _removeAtStatement) should have thier reference.

@ganicke @abchatra

[riknoll]

code looks good! however, please leave the remove-at-statement.md file.

our docs are served "live-ish" from this repo and so changing this now will break existing targets that don't have your change to the help annotation in pxt-core.d.ts. this file can be removed once this change has been propagated to other editors.

[ganicke]

The problem I have with combining these is that pop() returns a value and _popStatement() does not. This will either need to be noted in the common reference page or the seperate reference pages will need to remain.

[abchatra]

@ganicke is this good to merge?

[ganicke]

I've added my edits for the additional 'statement only' info.

removal of '_remove-at-statement.md' is reverted