docs: overriding multi-line usage

Extend Command::override_usage() doc string to describe how to format
multiple usage lines to be compatible with clap's default help formatter.

Relates-to: clap-rs#3413

src/builder/command.rs

@@ -1604,6 +1604,16 @@ impl Command {
     /// strings. After this setting is set, this will be *the only* usage string
     /// displayed to the user!
     ///
+    /// # Multiple usage lines
+    ///
+    /// Multiple usage lines may be present in the usage argument, but some
+    /// rules need to be followed to ensure the usage lines are formatted
+    /// correctly by the default help formatter:
+    ///
+    /// - Do not indent the first usage line.
+    /// - Indent all subsequent usage lines with four spaces.
+    /// - The last line must not end with a newline.
+    ///
     /// # Examples
     ///
     /// ```no_run
@@ -1612,6 +1622,33 @@ impl Command {
     ///     .override_usage("myapp [-clDas] <some_file>")
     /// # ;
     /// ```
+    ///
+    /// Or for multiple usage lines:
+    ///
+    /// ```no_run
+    /// # use clap::{Command, Arg};
+    /// Command::new("myprog").override_usage(
+    ///     "\
+    ///     myapp -X [-a] [-b] <file>
+    ///     myapp -Y [-c] <file1> <file2>
+    ///     myapp -Z [-d|-e]"
+    /// )
+    /// # ;
+    /// ```
+    ///
+    /// or equivalently, but with different string escaping:
+    ///
+    /// ```no_run
+    /// # use clap::{Command, Arg};
+    /// Command::new("myprog")
+    ///     .override_usage(
+    ///         "myapp -X [-a] [-b] <file>\n    \
+    ///          myapp -Y [-c] <file1> <file2>\n    \
+    ///          myapp -Z [-d|-e]"
+    ///     )
+    /// # ;
+    /// ```
+    ///
     /// [`ArgMatches::usage`]: ArgMatches::usage()
     #[must_use]
     pub fn override_usage(mut self, usage: impl Into<Str>) -> Self {