Move attributes.coverage.allowed-positions below syntax

Toss up which makes more sense in sequence, but this felt a little
better to me.

Author: ehuss

src/attributes/coverage-instrumentation.md

@@ -13,15 +13,15 @@ r[attributes.coverage]
 r[attributes.coverage.intro]
 The *`coverage` [attribute]* indicates whether a function should instrument code coverage at all and show up in code coverage reports.
 
-r[attributes.coverage.allowed-positions]
-The `coverage` attribute can only be controlled at the function level, but it can be applied to modules, `impl` blocks, or anything that can contain functions.
-
 r[attributes.coverage.syntax]
 There are two ways to use the coverage attribute:
 
 * `#[coverage(off)]` indicates that all functions within an item, recursively, should not be instrumented, unless specified by another attribute.
 * `#[coverage(on)]` (the default) indicates that all functions within an item, recursively, *should* be instrumented, unless specified by another attribute.
 
+r[attributes.coverage.allowed-positions]
+The `coverage` attribute can only be controlled at the function level, but it can be applied to modules, `impl` blocks, or anything that can contain functions.
+
 r[attributes.coverage.nesting]
 More-specific attributes always take priority over less-specific ones, e.g. if a crate is marked `#![coverage(off)]`, then functions inside that crate marked `#[coverage(on)]` will still have coverage.