README: FD caches are now cleared every 10 transactions

Also some other minor editorial changes.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

README.md

@@ -312,20 +312,24 @@ Unfixed kernel bugs (as of 4.11.9) with workarounds in Bees:
   hangs within a few hours, requiring a reboot to recover.  On the other
   hand, the `fsync()` only costs about 8% of overall performance.
 
-Not really a bug, but a gotcha nonetheless:
+Not really bugs, but gotchas nonetheless:
 
 * If a process holds a directory FD open, the subvol containing the
   directory cannot be deleted (`btrfs sub del` will start the deletion
   process, but it will not proceed past the first open directory FD).
   `btrfs-cleaner` will simply skip over the directory *and all of its
   children* until the FD is closed.  Bees avoids this gotcha by closing
-  all of the FDs in its directory FD cache every 15 minutes.
+  all of the FDs in its directory FD cache every 10 btrfs transactions.
 
 * If a file is deleted while Bees is caching an open FD to the file,
   Bees continues to scan the file.  For very large files (e.g. VM
   images), the deletion of the file can be delayed indefinitely.
   To limit this delay, Bees closes all FDs in its file FD cache every
-  15 minutes.
+  10 btrfs transactions.
+
+* If a snapshot is deleted, bees will generate a burst of exceptions
+  for references to files in the snapshot that no longer exist.  This
+  lasts until the FD caches are cleared.
 
 Installation
 ============
@@ -406,7 +410,7 @@ Dependencies
   can trigger known performance bugs and hangs in dedup-related functions.
 
   When in doubt, use a newer kernel version.  As of kernel 4.15.3 there
-  is no released Linux kernel that has no known bugs.
+  is no released Linux kernel that has no relevant known bugs.
 
 * markdown