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/android/fastboot.rst b/doc/android/fastboot.rst
index 16b1139..ce513a2 100644
--- a/doc/android/fastboot.rst
+++ b/doc/android/fastboot.rst
@@ -154,6 +154,10 @@
 controller, SD/MMC controller) or disk index. The partition index starts
 from ``1`` and describes the partition number on the particular device.
 
+Alternatively, partition types may be specified using :ref:`U-Boot's partition
+syntax <partitions>`. This allows specifying partitions like ``0.1``,
+``0#boot``, or ``:3``. The interface is always ``mmc``.
+
 Writing Partition Table
 -----------------------