docs: fix four dead links to the Multi Round-Trip Requests section

[tonydzi]

What was broken

Four links in docs/client.md point at a section that does not exist.

Three of them sit right where a reader needs them — at the end of the Roots, Sampling
and Elicitation sections, on the sentence explaining that from protocol version
2026-07-28 these requests are delivered via the Multi Round-Trip Requests pattern:

[Multi Round-Trip Requests](protocol.md#multi-round-trip-requests-mrtr)

protocol.md has no MRTR section. Following the link drops you at the top of a page that
never mentions the pattern — while the explanation you were promised is one screen further
down the same page you were already on (client.md → Multi Round-Trip Requests).

The fourth one is in that MRTR section itself, sending you to the server side:

[server-side documentation](server.md#multi-round-trip-requests-mrtr).

server.md does have the section, but its heading is ## Multi Round-Trip Requests, so the
slug is #multi-round-trip-requests — the trailing -mrtr is stale.

How I checked

Against the published site, not just the source:

$ curl -sL https://go.sdk.modelcontextprotocol.io/protocol | grep -o 'id="[^"]*round[^"]*"'
            # (no output)
$ curl -sL https://go.sdk.modelcontextprotocol.io/client  | grep -o 'id="[^"]*round[^"]*"'
id="multi-round-trip-requests"
$ curl -sL https://go.sdk.modelcontextprotocol.io/server  | grep -o 'id="[^"]*round[^"]*"'
id="multi-round-trip-requests"

So /protocol carries no anchor containing round at all, and both live targets use the
plain slug.

git log -S says the links arrived with b9e6fc5 ("mcp: add documentation for new protocol
version", #1019, 2026-06-23) and that protocol.src.md has never contained a
Multi Round-Trip Requests heading — the section landed in client.src.md and
server.src.md instead, and these four references were never repointed.

I also ran a small link/anchor checker over docs/: 4 bad anchors before, 0 after. (Two
"broken links" it also reports in server.md are its own false positives — Go generics
inside a fence, jsonschema.For[WeatherInput](opts), read as markdown link syntax. Not
touched.)

What it is now

The three per-feature links become same-page references, matching how client.md already
links its own sections ((see [Capabilities](#capabilities))):

[Multi Round-Trip Requests](#multi-round-trip-requests)

and the cross-page one drops the stale suffix:

[server-side documentation](server.md#multi-round-trip-requests).

Change is in internal/docs/client.src.md; docs/client.md regenerated with
go generate ./..., both committed together. Locally go generate ./... leaves the tree
clean afterwards, which is what docs-check asserts. Docs-only, four link targets, no prose
and no code paths touched.

---

Assisted-by: Claude Opus 5 — this PR was drafted and verified by Anton's AI cofounder running
on his account; every command and output above is from a real run on this machine, not a
reconstruction.