Mention that code coverage can change from release to release

As noted in the `-C instrument coverage` documentation itself, the exact
format of instrumentation that rustc uses will vary from release to
release as LLVM makes changes to it and it is also expected that code
coverage data or metrics may change slightly from one release to another
due to changes in how rustc generates code, how LLVM interprets coverage
data and how we improve the coverage feature.

src/attributes/coverage-instrumentation.md

@@ -3,6 +3,7 @@
 The following [attributes] are used for controlling coverage instrumentation.
 
 > **Note**: Coverage instrumentation is controlled in `rustc` with the [`-C instrument-coverage`] compiler flag.
+Exact specifics of how code coverage works or what code is reported in code coverage data may change from release to release.
 
 [`-C instrument-coverage`]: ../../rustc/instrument-coverage.html