rados/striper: add a package doc comment for striper

Add a package doc comment that also points out some things I learned
about how `rados` command line tool lists striped objects to preempt
being asked later on. :-)

Signed-off-by: John Mulligan <jmulligan@redhat.com>

rados/striper/doc.go

@@ -0,0 +1,22 @@
+//go:build ceph_preview
+
+/*
+Package striper contains a set of wrappers around Ceph's libradosstriper API.
+
+The Striper type supports synchronous operations to read and write data,
+as well as read and manipulate xattrs. Note that a striped object will
+consist of one or more objects in RADOS.
+
+There is no object list API in libradosstriper. Listing objects must be done
+using the base RADOS APIs. Striped objects will be stored in RADOS using the
+provided Striped Object ID (soid) suffixed by a dot (.) and a 16 byte
+0-prefixed hex number (for example, "foo.0000000000000000" or
+"bar.000000000000000a"). The object suffixed with ".0000000000000000" is the
+0-index stripe and will also possess striper specific xattrs (see the [ceph
+libradosstriper implementation] for a list) that are hidden from the
+libradosstriper xattr APIs.  You can use the name and/or these striper xattrs
+to distinguish a striped object from a non-striped RADOS object.
+
+[ceph libradosstriper implementation]: https://github.com/ceph/ceph/blob/2fa0e43b7e714df9811f87cbc5bf862ac503483c/src/libradosstriper/RadosStriperImpl.cc#L94-L97
+*/
+package striper