[doc] Hjson Usage and Style Guide says integers with commas aren't allowed. Clarify?

Nobody has claimed this yet.

https://github.com/lowRISC/opentitan/blob/a5f70f43bdd063877d68dbe81ef38a9acccca9ae/doc/contributing/style_guides/hjson_usage_style.md?plain=1#L118-L121

This isn't true; `tag: 2, ...` in hjson will define the field `tag` as the numeric value `2` and continue parsing other fields (whereas `tag: "2"` will make it the string `"2"`, with a similar effect but with type string rather than numeric).
On the other hand, `tag: 0x2, ...` will be interpreted as an unquoted string and "consume" the rest of the line since `0x2` is not a valid number format recognized by JSON, so the note of caution would apply there.

```python
>>> hjson.loads('''{ name: "fred", tag: 2, desc: "fred has tag 2" }''')
OrderedDict([('name', 'fred'), ('tag', 2), ('desc', 'fred has tag 2')])
>>> hjson.loads('''{ name: "fred", tag: "2", desc: "fred has tag 2" }''')
OrderedDict([('name', 'fred'), ('tag', '2'), ('desc', 'fred has tag 2')])
```

Does OpenTitan require strings in all "scalar" fields in hjson files? The documentation (for example [util/reggen/README.md](https://github.com/lowRISC/opentitan/blob/a5f70f43bdd063877d68dbe81ef38a9acccca9ae/util/reggen/README.md#configuration-and-register-definition-file-format)) doesn't make this clear, and explicitly states that certain fields are of type "int" (but they seem to be hjson strings containing an integer).
If this is the case, maybe it should be clarified somewhere that hjson files in OpenTitan use strings only, and not hjson numeric/boolean values.

Read doc/contributing/style_guides/hjson_usage_style.md around lines 118-121 and util/reggen/README.md under the configuration and register definition format section. Verify the documented scalar-value rules against the examples in the issue, then clarify the guidance so integer formatting and the expected types for OpenTitan Hjson fields are unambiguous.

Written by the indexing model from the issue text.