Improve std.experimental.allocator docs

[ntrel]

Add missing LREFs in top table.
Use RUNNABLE_EXAMPLE for synopsis.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19737	enhancement	[std.experimental.allocator] link table in docs has missing symbols

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8604"

[ntrel]

auto-tester failure must be unrelated (only doc changes here).

****** FAIL debug32 std.datetime.stopwatch
core.exception.AssertError@std/datetime/stopwatch.d(169): unittest failure