Documentation: RCU: Rename txt files to rst

Rename the following files to reST:
  - rcu.txt
  - listRCU.txt
  - UP.txt

Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.rst
similarity index 100%
rename from Documentation/RCU/UP.txt
rename to Documentation/RCU/UP.rst

diff --git a/Documentation/RCU/listRCU.txt b/Documentation/RCU/listRCU.rst
similarity index 100%
rename from Documentation/RCU/listRCU.txt
rename to Documentation/RCU/listRCU.rst

diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.rst
similarity index 100%
rename from Documentation/RCU/rcu.txt
rename to Documentation/RCU/rcu.rst