Clarify `parse_dot_suffix_expr`.

For the `MiddleDot` case, current behaviour:
- For a case like `1.2`, `sym1` is `1` and `sym2` is `2`, and `self.token`
  holds `1.2`.
- It creates a new ident token from `sym1` that it puts into `self.token`.
- Then it does `bump_with` with a new dot token, which moves the `sym1`
  token into `prev_token`.
- Then it does `bump_with` with a new ident token from `sym2`, which moves the
  `dot` token into `prev_token` and discards the `sym1` token.
- Then it does `bump`, which puts whatever is next into `self.token`,
  moves the `sym2` token into `prev_token`, and discards the `dot` token
  altogether.

New behaviour:
- Skips creating and inserting the `sym1` and dot tokens, because they are
  unnecessary.
- This also demonstrates that the comment about `Spacing::Alone` is
  wrong -- that value is never used. That comment was added in #77250,
  and AFAICT it has always been incorrect.

The commit also expands comments. I found this code hard to read
previously, the examples in comments make it easier.

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