Use RFC_2119 style for runtime_linux.md

Signed-off-by: Qiang Huang <[email protected]>

Author: hqhq

runtime-linux.md

@@ -1,8 +1,7 @@
 # Linux Runtime
 
 ## File descriptors
-By default, only the `stdin`, `stdout` and `stderr` file descriptors are kept open for the application by the runtime.
-
-The runtime may pass additional file descriptors to the application to support features such as [socket activation](http://0pointer.de/blog/projects/socket-activated-containers.html).
 
-Some of the file descriptors may be redirected to `/dev/null` even though they are open.
+By default, only the `stdin`, `stdout` and `stderr` file descriptors are kept open for the application by the runtime.
+The runtime MAY pass additional file descriptors to the application to support features such as [socket activation](http://0pointer.de/blog/projects/socket-activated-containers.html).
+Some of the file descriptors MAY be redirected to `/dev/null` even though they are open.