docs: use -cross instead of -os cross

vlib/v/checker/if.v

@@ -268,7 +268,7 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 			}
 			if c.fn_level == 0 && c.pref.output_cross_c {
 				// do not skip any of the branches for top level `$if OS {`
-				// statements, in `-os cross` mode
+				// statements, in `-cross` mode
 				found_branch = false
 				c.skip_flags = false
 				c.ct_cond_stack << branch.cond

vlib/v/doc/doc.v

@@ -46,7 +46,7 @@ pub enum Platform {
 	vinix
 	haiku
 	raw
-	cross // TODO: add functionality for v doc -os cross whenever possible
+	cross // TODO: add functionality for v doc -cross whenever possible
 }
 
 // copy of pref.os_from_string

vlib/v/help/build/build-c.txt

@@ -109,11 +109,13 @@ see also `v help build`.
          The module declaration should be `builtin`. The default Linux
          implementation can be found in `vlib/builtin/linux_bare`.
 
+   -cross
+      With `-cross`, V will attempt to output cross-platform C code.
+
    -os <os>, -target-os <os>
       Change the target OS that V tries to compile for.
       By default, the target OS is the host system.
-      When OS is `cross`, V will attempt to output cross-platform C code.
-
+
       Here is a list of the operating systems, supported by V:
       (CI tests runs on every commit/PR for each of these):
             `windows`, `linux`, `macos`
@@ -142,7 +144,7 @@ see also `v help build`.
       If you compile with `-os freebsd`, then x_default.c.v will be used.
       If you compile with `-os linux`, then x_linux.c.v will be used.
       If you compile with `-os windows`, then x_windows.c.v will be used.
-      If you compile with `-os cross`, then all, *except x_default.c.v*
+      If you compile with `-cross`, then all, *except x_default.c.v*
       will be used, wrapped in conditional compilation guards, so that
       the generated C source code will be larger, but will compile on all
       explicitly supported platforms without source changes.