Documentation reference shard_count heuristics

The [documentation for `shard_count`](https://docs.bazel.build/versions/main/be/common-definitions.html#common-attributes-tests) says:

> This value will override any heuristics used to determine the number of parallel shards with which to run the test. Note that for some test rules, this parameter may be required to enable sharding in the first place. Also see --test_sharding_strategy.

But the [docs](https://docs.bazel.build/versions/main/command-line-reference.html#flag--test_sharding_strategy) also say the only permitted values for `--test_sharding_strategy` are explicit and disabled. In other words, Bazel never shards a test that doesn't specify `shard_count`.

The reference to heuristics appears to be vestigial (or future?).

Start by comparing the shard_count section in common-definitions.html with the --test_sharding_strategy reference in command-line-reference.html. Confirm the documented Bazel behavior in the documentation source, then make the two descriptions consistent and verify the rendered references no longer imply unsupported heuristics.

Written by the indexing model from the issue text.