Expand zig fetch usage help doc to explain URL (#22850)

The current zig fetch help docs tell the user to specify a package's URL, but it's unclear what the URL should be.

This change expands the help output to explain what URLs the zig fetch command can handle and provides examples of valid URLs.

Related: #20096

A git bundle file seems to be the more accurate term, as it's what git uses in its documentation: https://git-scm.com/docs/git-bundle

Author: mtlynch

src/main.zig

@@ -6956,6 +6956,16 @@ const usage_fetch =
     \\Usage: zig fetch [options] <path>
     \\
     \\    Copy a package into the global cache and print its hash.
+    \\    <url> must point to one of the following:
+    \\      - A git+http / git+https server for the package
+    \\      - A tarball file (with or without compression) containing
+    \\        package source
+    \\      - A git bundle file containing package source
+    \\
+    \\Examples:
+    \\
+    \\  zig fetch --save git+https://example.com/andrewrk/fun-example-tool.git
+    \\  zig fetch --save https://example.com/andrewrk/fun-example-tool/archive/refs/heads/master.tar.gz
     \\
     \\Options:
     \\  -h, --help                    Print this help and exit