Clarify the work associated with tier 2 and tier 1 targets

Tier 2 and tier 1 targets place work on Rust project developers,
specifically; describe it as such rather than "the Rust community".

Also add the observation that the broader community may feel more
inclination to support higher-tier targets, but clarify that people are
not obligated to do so.

Author: joshtriplett

text/0000-target-tier-policy.md

@@ -47,8 +47,10 @@ Rust provides three tiers of target support:
 Adding a new tier 3 target imposes minimal requirements; we focus primarily on
 avoiding disruption to other ongoing Rust development.
 
-Tier 2 and tier 1 targets place work on the Rust community as a whole, to avoid
-breaking the target. Thus, these tiers require commensurate and ongoing efforts
+Tier 2 and tier 1 targets place work on Rust project developers as a whole, to
+avoid breaking the target. The broader Rust community may also feel more
+inclined to support higher-tier targets in their crates (though they are not
+obligated to do so). Thus, these tiers require commensurate and ongoing efforts
 from the maintainers of the target, to demonstrate value and to minimize any
 disruptions to ongoing Rust development.