[vcpkg-cmake] Fix docs and usage of option MAYBE_UNUSED_VARIABLES

[JackBoosY]

In the current document, our description of MAYBE_UNUSED_VARIABLES caused the user to misunderstand the parameter and incorporate the wrong changes.

Fix the option description and add sample code to the docs.

Related: #13467

[JackBoosY]

cc @wrobelda, can you please check MAYBE_UNUSED_VARIABLES's values in your opened PRs?

[github-actions]

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After committing all other changes, the version database must be updated

git add -u && git commit
git checkout fa1bbe097b26678e3fd992173b62279c071c422b -- versions
./vcpkg x-add-version --all

Diff

diff --git a/versions/v-/vcpkg-cmake.json b/versions/v-/vcpkg-cmake.json
index 87ddd66..086edd0 100644
--- a/versions/v-/vcpkg-cmake.json
+++ b/versions/v-/vcpkg-cmake.json
@@ -1,7 +1,7 @@
 {
   "versions": [
     {
-      "git-tree": "8d70d64792b10b091a27d19372db25cfb74d9e9c",
+      "git-tree": "c6df7e0faeda9e783175746c7c440c9bc2bdb111",
       "version-date": "2021-07-30",
       "port-version": 0
     },

[JackBoosY]

Need to check the related warning message after pipeline test.

[JackBoosY]

Question: should we also add the following requirement in https://github.com/microsoft/vcpkg/blob/master/docs/maintainers/maintainer-guide.md#build-techniques?