Extend coverage attribute documentation

This adds more details about the behavior of the `coverage` attribute,
along with some editorial changes.

Author: ehuss

src/attributes/coverage-instrumentation.md

@@ -11,10 +11,10 @@ The following [attributes] are used for controlling coverage instrumentation.
 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.
+The *`coverage` [attribute]* indicates whether a function should include instrumentation for code coverage and show up in code coverage reports.
 
 r[attributes.coverage.syntax]
-There are two ways to use the coverage attribute:
+The attribute uses the [_MetaListIdents_] syntax to specify its behavior:
 
 * `#[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.
@@ -35,10 +35,25 @@ impl S {
 ```
 
 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.
+The `coverage` attribute can only be controlled at the granularity of individual functions. It can be applied to [functions], [closures], [associated functions], [implementations], [modules], or [the crate root].
+
+It is an error to specify the attribute on a trait function without a body.
+
+r[attributes.coverage.trait-impl-inherit]
+When specified on a trait function, the attribute only applies to the default function body. Trait implementations do not inherit the setting from the trait definition.
+
+r[attributes.coverage.duplicates]
+It is an error to specify the `#[coverage]` attribute multiple times on the same item.
 
 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.
+Coverage attributes on more deeply nested items take priority over attributes at a higher nesting level. For example, if a crate is marked `#![coverage(off)]`, then functions inside that crate marked `#[coverage(on)]` will still have coverage.
 
+[_MetaListIdents_]: ../attributes.md#meta-item-attribute-syntax
+[associated functions]: ../items/associated-items.md#associated-functions-and-methods
 [attribute]: ../attributes.md
 [attributes]: ../attributes.md
+[closures]: ../expressions/closure-expr.md
+[functions]: ../items/functions.md
+[implementations]: ../items/implementations.md
+[modules]: ../items/modules.md
+[the crate root]: ../crates-and-source-files.md