Apply suggestions from code review

Co-authored-by: Jean-Christophe Morin <38703886+JeanChristopheMorinPerso@users.noreply.github.com>
Signed-off-by: Ben Andersen <156872503+isohedronpipeline@users.noreply.github.com>

src/rez/resolved_context.py

@@ -205,7 +205,7 @@ def __init__(self, package_requests, verbosity=0, timestamp=None,
             package_caching (bool|None): If True, apply package caching settings
                 as per the config. If None, enable as determined by config
                 setting :data:`package_cache_during_build`.
-            package_cache_async (Optional[bool]): If True, cache packages asynchronously.
+            package_cache_async (bool|None): If True, cache packages asynchronously.
                 If None, use the config setting :data:`package_cache_async`
         """
         self.load_path = None

src/rez/rezconfig.py

@@ -278,8 +278,10 @@
 # Enable package caching during a package build.
 package_cache_during_build = False
 
-# Enable package caching to run asynchronously during a resolve.
-# If this is false, a resolve will block until all packages are cached.
+# Asynchronously cache packages. If this is false, resolves will block until
+# all packages are cached.
+#
+# .. versionadded:: 2.1.0
 package_cache_async = True
 
 # Allow caching of local packages. You would only want to set this True for