bpo-33943: Add references in the docs for logging.basicConfig (GH-7858) (GH-7900)

Adds references to info about file modes, `time.strftime()`, string formatting
syntaxes, and logging levels.

(cherry picked from commit a8ddf85a84364d2c660e6670a7e06621993d1fdc)

Co-authored-by: Andrés Delfino <adelfino@gmail.com>
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 457e1c3..7b1b0ca 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -971,6 +971,11 @@
    encoding. (For reading and writing raw bytes use binary mode and leave
    *encoding* unspecified.)  The available modes are:
 
+   .. _filemodes:
+
+   .. index::
+      pair: file; modes
+
    ========= ===============================================================
    Character Meaning
    ========= ===============================================================