[flang][docs] Fix text and links in doxygen landing page

flang/docs/doxygen-mainpage.dox

@@ -3,13 +3,13 @@
 /// \section main_intro Introduction
 /// Welcome to the flang project.
 ///
-/// This documentation describes the **internal** software that makes
-/// up flang, not the **external** use of flang. There are no instructions
+/// The documentation here describes the **internal** software that makes
+/// up flang. There are no instructions
 /// here on how to use flang, only the APIs that make up the software. For
-/// usage instructions, please see the [project website](https://github.com/llvm-project/flang)
-/// for further detail.
+/// usage instructions, please consult flang's
+/// [user documentation](https://flang.llvm.org/docs/#using-flang).
 ///
 /// \section main_caveat Caveat
 /// This documentation is generated directly from the source code with doxygen.
-/// Since flang is constantly under active development, what you're about to
-/// read is out of date!
+/// Since flang is under active development, some of what you're about to read
+/// may already be out of date.