Document what a "subcommand" is

There are a few references to "subcommands" in the repository, but no
explanation of what they are. Beef up the docblock for
`shouldRunAsSubcommand()` to make things a little clearer.
Specifically, I'd like to make the difference between driver jobs and
driver subcomnmands clearer.

In addition, remove a double negative comment, "If we are not run as
'swift', don't do anything special", and replace it with something a
little clearer (in my opinion).

Author: modocache

tools/driver/driver.cpp

@@ -74,7 +74,11 @@ extern int swift_indent_main(ArrayRef<const char *> Args, const char *Argv0,
 extern int swift_symbolgraph_extract_main(ArrayRef<const char *> Args, const char *Argv0,
 void *MainAddr);
 
-/// Determine if the given invocation should run as a subcommand.
+/// Determine if the given invocation should run as a "subcommand".
+///
+/// Examples of "subcommands" are 'swift build' or 'swift test', which are
+/// usually used to invoke the Swift package manager executables 'swift-build'
+/// and 'swift-test', respectively.
 ///
 /// \param ExecName The name of the argv[0] we were invoked as.
 /// \param SubcommandName On success, the full name of the subcommand to invoke.