Rollup merge of #77449 - ssomers:btree_drain_filter_size_hint, r=Mark-Simulacrum

BTreeMap: comment why drain_filter's size_hint is somewhat pessimistic

The `size_hint` of the `DrainFilter` iterator doesn't adjust as you iterate. This hardly seems important to me, but there has been a comparable PR #64383 in the past. I guess a scenario is that you first iterate half the map manually and keep most of the key/value pairs in the map, and then tell the predicate to drain most of the key/value pairs and `.collect` the iterator over the remaining half of the map.

I am totally ambivalent whether this is better or not.

r? @Mark-Simulacrum

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