DOC: Clarify .agg status

[dsm054]

The documentation for .agg is almost hilariously ambiguous:

agg is an alias for aggregate. Use it.

What is "it" referring to? Are we being told to use aggregate instead of agg because it's the fundamental name and agg is a mere alias? Or are we being told that we can and should use agg because it's the canonical alias (in the same way that pd is the canonical alias for pandas)?

A quick scan of the codebase suggests that .agg is considerably more popular than .aggregate among pandas devs, FWIW..

[gfyoung]

The grammarian says "aggregate" because the last noun before the pronoun is "aggregate"

However, the correct reference is agg, which is why you see .agg being used more often.