docs: document pre_srpm_build and umount_root hooks

Author: praiskup

docs/Plugin-Hooks.md

@@ -28,10 +28,12 @@ Where `HOOK_NAME` is one of:
 * prechroot - This hook is called just before executing a command when `mock chroot` command has been used. This action is currently being used by `root_cache` plugin.
 * preinit - Only chroot and result_dir directories exist. There may be no other content. No binary. Not even base directories. Root_cache plugin uses this hook to populate the content of chroot_from cache. HW_info gather information about hardware and store it in result_dir. This action is currently being used by `ccache`, `hw_info`, `root_cache`, `yum_cache` plugins.
 * preshell - This hook is called just before giving a prompt to a user when `mock shell` command has been used. This action is currently being used by `pm_request`, `root_cache` plugins.
+* pre_srpm_build - Called before generating the SRPM (`rpmbuild -bs`).  Used in `--buildsrpm` mode only.
 * preyum - This hook is called before any packager manager action is executed. This is, e.g., used by `root_cache` plugin when `--cache-alterations` option is used. This action is currently being used by `root_cache`, `selinux`, `yum_cache`.
 * process_logs - Called once the build log files (root.log, build.log, ...) are completed so they can be processed (e.g. compressed by the `compress_logs` plugin).
 * remove_snapshot
 * rollback_to
 * scrub
+* umount_root - The counterpart to mount_root; called to instruct the buildroot-managing plugin to unmount the buildroot.
 
 You can get inspired by existing [plugins](https://github.com/rpm-software-management/mock/tree/master/mock/py/mockbuild/plugins).