doc: Add documentation for the genalloc subsystem

Genalloc/genpool has kerneldoc comments, but nothing has ever been pulled
into the docs themselves.  Here's a first attempt, repurposed from an
article I wrote at https://lwn.net/Articles/729653/.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 0606be3..d5bbe03 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -20,6 +20,7 @@
    genericirq
    flexible-arrays
    librs
+   genalloc
 
 Interfaces for kernel debugging
 ===============================