Document silent uninstallation of NVDA in the user guide (#17203)

Closes #16966

Summary of the issue:
While it is possible to silently uninstall NVDA from the command line, this option is not documented anywhere.

Description of user facing changes:
Added notes on command line uninstallation of NVDA to the "Command Line Options" section of the user guide.

Description of development approach:
Read the NSIS documentation to ensure there wasn't any nuance I was missing.

Testing strategy:
Tested silently uninstalling NVDA.

Known issues with pull request:
Users may find it confusing that the installer uses Unix-style options, while the uninstaller uses DOS-style options.

user_docs/en/userGuide.md

@@ -5290,6 +5290,16 @@ Following are the command line options for NVDA:
 |None |`--create-portable-silent` |Creates a portable copy of NVDA  (without starting the new copy). Requires `--portable-path` to be specified. This option suppresses warnings when writing to non-empty directories and may overwrite files without warning.|
 |None |`--portable-path=PORTABLEPATH` |The path where a portable copy will be created|
 
+Just as you can silently install NVDA by passing the `--install-silent` command line option to NVDA, it can be silently uninstalled by passing the `/S` command to the uninstaller.
+
+NVDA's uninstaller is called `uninstall.exe` and resides under the NVDA installation directory, `%ProgramFiles(x86)%\nvda` on 64-bit Windows, or `%ProgramFiles%\nvda` on 32-bit Windows.
+
+The following are the command line options for NVDA's uninstaller:
+
+| Short |Long |Description|
+|---|---|---|
+|`/S` |None |Silently uninstall NVDA. |
+
 ### System Wide Parameters {#SystemWideParameters}
 
 NVDA allows some values to be set in the system registry which alter the system wide behaviour of NVDA.