JuliaSyntax: Add a brief docstring for JuliaSyntax.build_tree

[DilumAluthge]

JuliaSyntax/docs/src/api.md currently has the following:

julia/JuliaSyntax/docs/src/api.md

Lines 19 to 23 in c93f0c8

	```@docs
	JuliaSyntax.parse!
	JuliaSyntax.ParseStream
	JuliaSyntax.build_tree
	```

But JuliaSyntax.build_tree doesn't currently have a docstring, so the docs build will fail. This PR adds a minimal docstring.

🤖 Assisted-by: Claude Code:Opus 5

[DilumAluthge]

@mlechu @topolarity What do you think of creating a docstring for build_tree, vs just removing build_tree from the @docs blocks. I think either would be sufficient for the purposes of fixing the docs build.

[DilumAluthge]

(IIUC, the JuliaSyntax repo set this kind of thing to be a warning when building the docs, but in the Julia repo, I think we make it a hard error that fails the docs build.)

[mlechu]

I'd prefer either removal of build_tree from the docs block, or a single docstring above function build_tree end (something like "Build a tree of type type from a raw parsed stream ps. This is the final step in parsing."

[DilumAluthge]

Sounds good. Do you have a preference between those two options?

[mlechu]

No strong preference, just remove it if you're unsure. I think most consumers call some string->T driver function instead of doing the ParseStream -> parse! -> build_tree dance

[DilumAluthge]

It looks like some other docstrings have @refs to build_tree, so I ended up going with the "single docstring" approach.