Add some explanations for frequently used rustbot commands (rust-lang#1849)

* add some explain for rustbot commands

* add more details about shortcuts

* fix words on `r=someone`

Co-authored-by: Yuki Okushi <[email protected]>

---------

Co-authored-by: Yuki Okushi <[email protected]>

Author: chenyukang

Diff for: src/contributing.md

@@ -255,7 +255,7 @@ If you encounter merge conflicts or when a reviewer asks you to perform some
 changes, your PR will get marked as `S-waiting-on-author`. When you resolve
 them, you should use `@rustbot` to mark it as `S-waiting-on-review`:
 
-    @rustbot label -S-waiting-on-author +S-waiting-on-review
+    @rustbot ready
 
 GitHub allows [closing issues using keywords][closing-keywords]. This feature
 should be used to keep the issue tracker tidy. However, it is generally preferred

Diff for: src/rustbot.md

@@ -44,9 +44,12 @@ the `@rustbot` command will look like this:
     @rustbot label -S-waiting-on-author +S-waiting-on-review
 
 The syntax for this command is pretty loose, so there are other variants of this
-command invocation. For more details, see [the docs page about labeling][labeling].
+command invocation. There are also some shortcuts to update labels,
+for instance `@rustbot ready` will do the same thing with above command.
+For more details, see [the docs page about labeling][labeling] and [shortcuts][shortcuts].
 
 [labeling]: https://forge.rust-lang.org/triagebot/labeling.html
+[shortcuts]: https://forge.rust-lang.org/triagebot/shortcuts.html
 
 ## Other commands
 

Diff for: src/walkthrough.md

@@ -186,10 +186,19 @@ request that reviewer by leaving a comment on the thread with `r?
 @reviewer-github-id` (e.g. `r? @eddyb`). If you don't know who to request,
 don't request anyone; the bot will assign someone automatically based on which files you changed.
 
-The reviewer may request changes before they approve your PR. Feel free to ask
-questions or discuss things you don't understand or disagree with. However,
+The reviewer may request changes before they approve your PR, they may mark the PR with label 
+"S-waiting-on-author" after leaving comments, this means that the PR is blocked on you to make 
+some requested changes. When you finished iterating on the changes, you can mark the PR as 
+`S-waiting-on-review` again by leaving a comment with `@rustbot ready`, this will remove the 
+`S-waiting-on-author` label and add the `S-waiting-on-review` label.
+
+Feel free to ask questions or discuss things you don't understand or disagree with. However,
 recognize that the PR won't be merged unless someone on the Rust team approves
-it.
+it. If a reviewer leave a comment like `r=me after fixing ...`, that means they approve the PR and 
+you can merge it with comment with `@bors r=reviwer-github-id`(e.g. `@bors r=eddyb`) to merge it 
+after fixing trivial issues. Note that `r=someone` requires permission and bors could say 
+something like "🔑 Insufficient privileges..." when commenting `r=someone`. In that case, 
+you have to ask the reviewer to revisit your PR.
 
 When your reviewer approves the PR, it will go into a queue for yet another bot
 called `@bors`. `@bors` manages the CI build/merge queue. When your PR reaches