net/http: fix doc comment on FormValue function

[eduardbme]

This function checks Request.Form, which now includes values parsed from a PATCH request.

Fixes #60585

[gopherbot]

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[gopherbot]

Message from David Chase:

Patch Set 1: Code-Review+2

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[gopherbot]

Message from Damien Neil:

Patch Set 1: Auto-Submit+1 Code-Review+2 Run-TryBot+1

(1 comment)

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Eduard Bondarenko:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR is being closed because golang.org/cl/513435 has been merged.