mm/gfp: add kernel-doc for gfp_t

The generated html will link to the definition of the gfp_t automatically
once we define it.  Move the one-paragraph overview of GFP flags from the
documentation directory into gfp.h and pull gfp.h into the documentation.

This generates warnings with clang
(https://lkml.kernel.org/r/20210219195509.GA59987@24bbad8f3778), so
use a #if 0 to hide it from the compiler for now.

Link: https://lkml.kernel.org/r/20210215204909.3824509-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20210220003049.GZ2858050@casper.infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/mm-api.rst

@@ -19,11 +19,8 @@ User Space Memory Access
 Memory Allocation Controls
 ==========================
 
-Functions which need to allocate memory often use GFP flags to express
-how that memory should be allocated. The GFP acronym stands for "get
-free pages", the underlying memory allocation function. Not every GFP
-flag is allowed to every function which may allocate memory. Most
-users will want to use a plain ``GFP_KERNEL``.
+.. kernel-doc:: include/linux/gfp.h
+   :internal:
 
 .. kernel-doc:: include/linux/gfp.h
    :doc: Page mobility and placement hints

include/linux/gfp.h

@@ -8,6 +8,20 @@
 #include <linux/linkage.h>
 #include <linux/topology.h>
 
+/* The typedef is in types.h but we want the documentation here */
+#if 0
+/**
+ * typedef gfp_t - Memory allocation flags.
+ *
+ * GFP flags are commonly used throughout Linux to indicate how memory
+ * should be allocated.  The GFP acronym stands for get_free_pages(),
+ * the underlying memory allocation function.  Not every GFP flag is
+ * supported by every function which may allocate memory.  Most users
+ * will want to use a plain ``GFP_KERNEL``.
+ */
+typedef unsigned int __bitwise gfp_t;
+#endif
+
 struct vm_area_struct;
 
 /*