Add a note about temporary directories to --build help text (#4944)

Document that an initial build occurs in a temporary directory, even when --build is specified.

Author: 1byxero

news/4262.doc

@@ -0,0 +1 @@
+Added documentation on usage of --build command line option

src/pip/_internal/cmdoptions.py

@@ -437,7 +437,10 @@ def only_binary():
     '-b', '--build', '--build-dir', '--build-directory',
     dest='build_dir',
     metavar='dir',
-    help='Directory to unpack packages into and build in.'
+    help='Directory to unpack packages into and build in. Note that '
+         'an initial build still takes place in a temporary directory. '
+         'The location of temporary directories can be controlled by setting '
+         'the TMPDIR environment variable (TEMP on Windows) appropriately.'
 )  # type: Any
 
 ignore_requires_python = partial(