doc: document purging upspinserver's storage
doc: document purging upspinserver's storage

Change-Id: Ia5ffbc4e53f1014910f2b0a746be22d54bde0a5d
Reviewed-on: https://upspin-review.googlesource.com/8270
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>

upspin/upspin,upspin/upspin