Clarify how to quote and respond to the target tier policy requirements

Several times, people have seemed unclear on how to respond to some of
the policy requirements, particularly those that just state things the
target developers must *not* do (e.g. not posting to PRs that break the
target). Add a note that such requirements just need acknowledgement,
nothing more.

Make quoting and responding a "must" rather than an "is encouraged to",
since it's easier to review the requirements that way.

Author: joshtriplett

src/doc/rustc/src/target-tier-policy.md

@@ -62,8 +62,10 @@ not preclude an existing target's maintainers using issues (on the Rust
 repository or otherwise) to track requirements that have not yet been met, as
 appropriate; however, before officially proposing the introduction or promotion
 of a target, it should meet all of the necessary requirements. A target
-proposal is encouraged to quote the corresponding requirements verbatim as part
-of explaining how the target meets those requirements.
+proposal must quote the corresponding requirements verbatim and respond to them
+as part of explaining how the target meets those requirements. (For the
+requirements that simply state that the target or the target developers must
+not do something, it suffices to acknowledge the requirement.)
 
 For a list of all supported targets and their corresponding tiers ("tier 3",
 "tier 2", "tier 2 with host tools", "tier 1", or "tier 1 with host tools"), see