Bug 1243603 - Post: Add stub documentation for command line parameters. r=Gijs

The small idea here is to set down some notes about `-osint`.  The
larger idea is to provide a location for future improvements.

Differential Revision: https://phabricator.services.mozilla.com/D132544

browser/docs/CommandLineParameters.rst

@@ -0,0 +1,29 @@
+=======================
+Command Line Parameters
+=======================
+
+Firefox recognizes many (many!) command line parameters.  Overview
+documentation of these parameters lives here.
+
+Known parameters
+----------------
+
+.. list-table::
+   :widths: 20 80
+   :header-rows: 1
+
+   * - Parameter
+     - Description
+   * - ``-osint``
+     - On Windows, ``-osint`` serves two purposes.  Most importantly, it signals
+       that the command line is untrusted and must be sanitized.  Command lines
+       with ``-osint`` are rejected unless they are of exactly the form
+       ``firefox.exe -osint -url URL``: refer to `the EnsureCommandLineSafe
+       function
+       <https://searchfox.org/mozilla-central/rev/ead7da2d9c5400bc7034ff3f06a030531bd7e5b9/toolkit/xre/CmdLineAndEnvUtils.h#196>`_.
+       These command lines are produced by apps delegating to Firefox, and the
+       resulting URL may not be correctly quoted.  The sanitization process
+       ensures that maliciously chosen URLs do not add additional parameters to
+       Firefox.  Secondarily, the ``-osint`` parameter signals that Firefox is
+       being invoked by Windows to handle a URL: generally a registered file
+       type, e.g., ``.html``, or protocol, e.g., ``https``.

browser/docs/index.rst

@@ -10,6 +10,7 @@ This is the nascent documentation of the Firefox front-end code.
    urlbar/index
    /toolkit/mozapps/update/docs/BackgroundUpdates
    BrowserUsageTelemetry
+   CommandLineParameters
    components/enterprisepolicies/docs/index
    extensions/formautofill/docs/index
    components/newtab/docs/index