doc: Document partition specifications

This documents the way U-Boot understands partitions specifications.
This also updates the fastboot documentation for the changes in the
previous commit.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index a8842bf..09372d4 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -6,6 +6,7 @@
 
    fdt_overlays
    netconsole
+   partitions
 
 Shell commands
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