Tweak/correct partial types and methods

[RexJaeschke]

While looking at the spec for the (V13) feature that adds partial properties and indexers, I had need to review the current spec for partial types and members. When doing so, I came up with some improvements and corrections to that, which this PR addresses.

Here's what this PR does:

1. Changes w.r.t to uses of can/must/may/shall.
2. Changes the title of 15.2.7, "Partial declarations" to "Partial type declarations" as that section is only about partial types and not at all about partial methods.
3. Corrects 15.3 Class members, which currently forbids partial methods. As well as fixing the words, I added a partial method to the accompanying example.
4. As the Documentation comments chapter said nothing about the handling of doc comments on partial elements, I added such text. Note than when multiple doc comments on an element are written to the document file, I've made their order unspecified. (They might be processed lexically within the same part, or lexically within multiple parts within in the same compilation unit, or who knows in what order if they span multiple compilation units.)

Note that V9 adds support for explicit accessor modifiers on partial members, and V13 adds support for partial properties and indexers. The edits proposed here will support (and not get in the way of) these future features.

[BillWagner]

I think this is close, but some of the comments require discussion

[BillWagner]

2025/02/19 meeting: After Rex addresses remaining comments, this is ready to merge.

[RexJaeschke]

@jskeet @BillWagner I think we're good to merge this.

[BillWagner]

This LGTM.