docs: add Angular CLI autocompletion doc

This explains how to set up autocompletion with the Angular CLI:

1. Via the automated prompt on first execution.
2. Via `ng completion`.
3. Manually via appending `source <(ng completion script)` to the user's `~/.bashrc` file.

It also specifically calls out supported and unsupported platforms as well as the requirement for a global install of the Angular CLI.

Author: dgp1130

packages/angular/cli/src/commands/completion/long-description.md

@@ -1,5 +1,73 @@
-To enable Bash and Zsh real-time type-ahead autocompletion, run
-`ng completion` and restart your terminal.
+Setting up autocompletion configures your terminal, so pressing the `<TAB>` key while in the middle
+of typing will display various commands and options available to you. This makes it very easy to
+discover and use CLI commands without lots of memorization.
 
-Alternatively, append `source <(ng completion script)` to the appropriate `.bashrc`,
-`.bash_profile`, `.zshrc`, `.zsh_profile`, or `.profile` file.
+![A demo of Angular CLI autocompletion in a terminal. The user types several partial `ng` commands,
+using autocompletion to finish several arguments and list contextual options.
+](/generated/images/guide/cli/completion.gif)
+
+## Automated setup
+
+The CLI should prompt and ask to set up autocompletion for you the first time you use it (v14+).
+Simply answer "Yes" and the CLI will take care of the rest.
+
+```
+$ ng serve
+? Would you like to enable autocompletion? This will set up your terminal so pressing TAB while
+typing Angular CLI commands will show possible options and autocomplete arguments. (Enabling
+autocompletion will modify configuration files in your home directory.) Yes
+Appended `source <(ng completion script)` to `/home/my-username/.bashrc`. Restart your terminal or
+run:
+
+source <(ng completion script)
+
+to autocomplete `ng` commands.
+
+# Serve output...
+```
+
+If you already refused the prompt, it won't ask again. But you can run `ng completion` to
+do the same thing automatically.
+
+This modifies your terminal environment to load Angular CLI autocompletion, but can't update your
+current terminal session. Either restart it or run `source <(ng completion script)` directly to
+enable autocompletion in your current session.
+
+Test it out by typing `ng ser<TAB>` and it should autocomplete to `ng serve`. Ambiguous arguments
+will show all possible options and their documentation, such as `ng generate <TAB>`.
+
+## Manual setup
+
+Some users may have highly customized terminal setups, possibly with configuration files checked
+into source control with an opinionated structure. `ng completion` only ever appends Angular's setup
+to an existing configuration file for your current shell, or creates one if none exists. If you want
+more control over exactly where this configuration lives, you can manually set it up by having your
+shell run at startup:
+
+```bash
+source <(ng completion script)
+```
+
+This is equivalent to what `ng completion` will automatically set up, and gives power users more
+flexibility in their environments when desired.
+
+## Platform support
+
+Angular CLI supports autocompletion for the Bash and Zsh shells on MacOS and Linux operating
+systems.
+
+Windows does not support autocompletion in native shells, such as Cmd and Powershell. However,
+the Angular CLI supports Git Bash and
+[Windows Subsystem for Linux](https://docs.microsoft.com/en-us/windows/wsl/) using Bash or Zsh.
+
+## Global install
+
+Autocompletion works by configuring your terminal to invoke the Angular CLI on startup to load the
+setup script. This means the terminal must be able to find and execute the Angular CLI, typically
+through a global install that places the binary on the user's `$PATH`. If you get
+`command not found: ng`, make sure the CLI is installed globally which you can do with the `-g`
+flag:
+
+```bash
+npm install -g @angular/cli
+```