Rollup merge of rust-lang#38013 - wezm:simplify-test-notes, r=steveklabnik

Simplify notes on testing and concurrency

The start of the notes on tests running concurrently, added in rust-lang#37766 read a little awkwardly. This PR fixes that and simplifies the wording a bit.

r? @steveklabnik

Author: steveklabnik

src/doc/book/testing.md

@@ -589,11 +589,10 @@ please see the [Documentation chapter](documentation.html).
 
 # Testing and concurrency
 
-One thing that is important to note when writing tests is that they may be run
-concurrently using threads. For this reason you should take care that your tests
-are written in such a way as to not depend on each-other, or on any shared
-state. "Shared state" can also include the environment, such as the current
-working directory, or environment variables.
+It is important to note that tests are run concurrently using threads. For this
+reason, care should be taken to ensure your tests do not depend on each-other,
+or on any shared state. "Shared state" can also include the environment, such
+as the current working directory, or environment variables.
 
 If this is an issue it is possible to control this concurrency, either by
 setting the environment variable `RUST_TEST_THREADS`, or by passing the argument