Document encoding detection for unknown encodings. (#2733)

Sometimes, cchardet might detect encodings that Python doesn't know. In
such cases, `.text()` function might raise a `LookupError`, and
`get_encoding` may return values that are unsafe to pass to `bytes.decode()`
or to `.text()` functions.

Closes #2732

CHANGES/2732.doc

@@ -0,0 +1 @@
+Document behavior when cchardet detects encodings that are unknown to Python.

docs/client_reference.rst

@@ -1170,6 +1170,9 @@ Response object
 
       :return str: decoded *BODY*
 
+      :raise LookupError: if the encoding detected by chardet or cchardet is
+                          unknown by Python (e.g. VISCII).
+
       .. note::
 
          If response has no ``charset`` info in ``Content-Type`` HTTP
@@ -1223,6 +1226,10 @@ Response object
       are no appropriate codecs for encoding then :term:`cchardet` /
       :term:`chardet` is used.
 
+      Beware that it is not always safe to use the result of this function to
+      decode a response. Some encodings detected by cchardet are not known by
+      Python (e.g. VISCII).
+
       .. versionadded:: 3.0