Improve sizehint! docstring about shrink arg

[nalimilan]

See #51929 (comment).

[matthias314]

Since we are talking about the docstring for sizehint!, I would like to comment on the sentence

If `first` is true, then the reserved space is from the start of the collection, for ordered
collections.

(which is not changed at present). I don't find the phrase "reserved from the start of the collection" very clear. What about the following:

If the collection is ordered and `first` is `true`, then any additional space is reserved
before the start of the collection rather than after the end. This way, subsequent calls
to `pushfirst!` (instead of `push!`) may become faster.

[nalimilan]

I've pushed a commit to apply the proposed changes.