Document format of .bazelversion file

Is there any documentation on the syntax supported in `.bazelversion` files?

Is it legal to use comments?
```
# Bazel 6 causes regression, fix to 5.x for now.
5.4.0
```

Can we specify sets of versions somehow, e.g. like this:
```
5.x.x
```
or
```
5
```

Would be great to have this be documented.

Start by locating the `.bazelversion` parser and any existing user documentation for Bazelisk. Document the supported syntax, including whether comments and version patterns such as `5.x.x` or `5` are accepted, and ensure the examples answer the questions raised in this issue.

Written by the indexing model from the issue text.