Linux: Restore commetions in <kernel-features.h>

2017-04-20  Florian Weimer  <fweimer@redhat.com>

	* sysdeps/unix/sysv/linux/kernel-features.h: Restore the full
	*ACCEPT4* macro documentation and apply it to the *RECVMMSG*
	macros.

diff --git a/sysdeps/unix/sysv/linux/kernel-features.h b/sysdeps/unix/sysv/linux/kernel-features.h
index b981466..f02b602 100644
--- a/sysdeps/unix/sysv/linux/kernel-features.h
+++ b/sysdeps/unix/sysv/linux/kernel-features.h
@@ -88,8 +88,17 @@
 #define __ASSUME_PREADV	1
 #define __ASSUME_PWRITEV	1
 
-/* Support for recvmmsg functionality was added in 2.6.33.  The macros
-   defined correspond to those for accept4.  */
+/* Support for recvmmsg functionality was added in 2.6.33, but for
+   some architectures using a separate syscall rather than socketcall
+   that syscall was only added later, and some architectures first had
+   socketcall support then a separate syscall.  Define
+   __ASSUME_RECVMMSG_SOCKETCALL if glibc uses socketcall on this
+   architecture and recvmmsg is available through socketcall,
+   __ASSUME_RECVMMSG_SYSCALL if it is available through a separate
+   syscall, __ASSUME_RECVMMSG_SYSCALL_WITH_SOCKETCALL if it became
+   available through a separate syscall at the same time as through
+   socketcall, and __ASSUME_RECVMMSG if the recvmmsg function is known
+   to work.  */
 #if __LINUX_KERNEL_VERSION >= 0x020621
 # ifdef __ASSUME_SOCKETCALL
 #  define __ASSUME_RECVMMSG_SOCKETCALL	1
@@ -115,7 +124,7 @@
 #endif
 
 /* Support for sendmmsg functionality was added in 3.0.  The macros
-   defined correspond to those for accept4 and recvmmsg.  */
+   defined correspond to those for recvmmsg.  */
 #if __LINUX_KERNEL_VERSION >= 0x030000
 # ifdef __ASSUME_SOCKETCALL
 #  define __ASSUME_SENDMMSG_SOCKETCALL	1