docs(resolver): Move Constraints context under new Constraints header

Author: epage

src/doc/src/reference/resolver.md

@@ -5,6 +5,14 @@ use based on the version requirements specified in each package. This process
 is called "dependency resolution" and is performed by the "resolver". The
 result of the resolution is stored in the `Cargo.lock` file which "locks" the
 dependencies to specific versions, and keeps them fixed over time.
+The [`cargo tree`] command can be used to visualize the result of the
+resolver.
+
+[dependency specifications]: specifying-dependencies.md
+[dependency specification]: specifying-dependencies.md
+[`cargo tree`]: ../commands/cargo-tree.md
+
+## Constraints and Heuristics
 
 In many cases there is no single "best" dependency resolution.
 The resolver operates under various constraints and heuristics to find a generally applicable resolution.
@@ -65,15 +73,6 @@ Key steps:
   falling back to the next version when backtracking.
   The resolver cannot backtrack because a more-preferred version becomes available.
 
-The [`cargo tree`] command can be used to visualize the result of the
-resolver.
-
-[dependency specifications]: specifying-dependencies.md
-[dependency specification]: specifying-dependencies.md
-[`cargo tree`]: ../commands/cargo-tree.md
-
-## Constraints and Heuristics
-
 ### SemVer compatibility
 
 Cargo uses [SemVer] for specifying version numbers. This establishes a common