[NFC] FunctionSamples::getEntrySamples -> getHeadSamplesEstimate

The name `getEntrySamples` was misleading for 2 reasons. One, it's
close in name to `Function::getEntryCount`, but the equivalent here is
`getHeadSamples`; second, as opposed to the other get* APIs in
`FunctionSamples`, it performs an estimate/heuristic rather than just
retrieving raw data (or a non-heuristic derivate off that data, like
`getMaxCountInside`)

The new name should more clearly communicate its intent; and, being
close (in name) to `getHeadSamples`, it should allow the reader discover
the relation between them.

Also updated the doc comments for both `getHeadSamples[Estimate]` so a
reader may better understand the relation between them.

Differential Revision: https://reviews.llvm.org/D130281