gh-122862: Fix description of the GC allocation counter in set_threshold() docs

[owenthcarey]

The gc.set_threshold() documentation says the collector tracks "the number of allocations minus the number of deallocations" since the last collection. The implementation doesn't quite do that: the generation 0 counter is incremented on each allocation and decremented on each deallocation, but it's never decremented below zero, and it's reset when a collection runs. This means deallocations that happen before allocations don't offset them, as described in the issue.

This rewrites the sentence to describe the actual behavior. It also fixes the missing "of" in "the number object allocations".

Fixes #122862

• Issue: GC: Implementation does not match behavior documented on gc.set_threshold #122862

[read-the-docs-community]

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± library/gc.html