stdlib: document float map key encoding in json

[mjc]

The json encoder has accepted Erlang maps with float keys since the original json module was added in 66a771e (Add json module). The runtime key dispatcher already handles floats with key(Key, _Encode) when is_float(Key) -> [$", encode_float(Key), $"], which emits the key as a JSON object name.

JSON object names are strings, so this does not add non-string keys to JSON. It updates the public encode_map/1 type and the encode/1 data mapping table to describe the existing Erlang input behavior, and adds encode tests for both the default and checked map encoding paths.

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CT Test Results

    2 files    100 suites   1h 0m 23s ⏱️
2 299 tests 2 247 ✅ 52 💤 0 ❌
2 718 runs  2 662 ✅ 56 💤 0 ❌

Results for commit 8a59d51.

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// Erlang/OTP Github Action Bot

[mjc]

force push: I had accidentally had 2 commits which is silly for a mostly-documentation pr.