HBASE-22628 Document the custom WAL directory (hbase.wal.dir) usage

Signed-off-by Reid Chan <reidchan@apache.org>

src/main/asciidoc/_chapters/architecture.adoc

@@ -2491,6 +2491,26 @@ For example: If source cluster FS client configurations are copied to the destin
 
 NOTE: `DefaultSourceFSConfigurationProvider` supports only `xml` type files. It loads source cluster FS client configuration only once, so if source cluster FS client configuration files are updated, every peer(s) cluster RS must be restarted to reload the configuration.
 
+[[arch.custom.wal.dir]]
+=== Custom WAL Directory
+HBASE-17437 added support for specifying a WAL directory outside the HBase root directory or even in a different FileSystem since 1.3.3/2.0+. Some FileSystems (such as Amazon S3) don’t support append or consistent writes, in such scenario WAL directory needs to be configured in a different FileSystem to avoid loss of writes.
+
+Following configurations were added to accomplish this:
+. `hbase.wal.dir`
++
+This defines where the root WAL directory is located, could be on a different FileSystem than the root directory. WAL directory can not be set to a subdirectory of the root directory. The default value of this is the root directory if unset.
++
+. `hbase.rootdir.perms`
++
+Configures FileSystem permissions to set on the root directory. This is '700' by default.
++
+. `hbase.wal.dir.perms`
++
+Configures FileSystem permissions to set on the WAL directory FileSystem. This is '700' by default.
++
+
+NOTE: While migrating to custom WAL dir (outside the HBase root directory or a different FileSystem) existing WAL files must be copied manually to new WAL dir, otherwise it may lead to data loss/inconsistency as HMaster has no information about previous WAL directory.
+
 [[arch.hdfs]]
 == HDFS