Merge pull request #1076 from github/rashiq-update_gh_ost_hook_docs

Author: rashiq

doc/hooks.md

@@ -68,6 +68,7 @@ The following variables are available on all hooks:
 - `GH_OST_INSPECTED_LAG` - lag in seconds (floating point) of inspected server
 - `GH_OST_HEARTBEAT_LAG` - lag in seconds (floating point) of heartbeat
 - `GH_OST_PROGRESS` - progress pct ([0..100], floating point) of migration
+- `GH_OST_ETA_SECONDS` - estimated duration until migration finishes in seconds
 - `GH_OST_MIGRATED_HOST`
 - `GH_OST_INSPECTED_HOST`
 - `GH_OST_EXECUTING_HOST`