internal/syscall/unix: document randomTrap

Updates #10848

Change-Id: I8353100ed01cb0e8fc19225157f5709bae388612
Reviewed-on: https://go-review.googlesource.com/20975Reviewed-by: Rob Pike <r@golang.org>

src/internal/syscall/unix/getrandom_linux_386.go

@@ -4,4 +4,6 @@
 
 package unix
 
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
 const randomTrap uintptr = 355

src/internal/syscall/unix/getrandom_linux_amd64.go

@@ -4,4 +4,6 @@
 
 package unix
 
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
 const randomTrap uintptr = 318

src/internal/syscall/unix/getrandom_linux_arm.go

@@ -4,4 +4,6 @@
 
 package unix
 
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
 const randomTrap uintptr = 384

src/internal/syscall/unix/getrandom_linux_generic.go

@@ -6,4 +6,11 @@
 
 package unix
 
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
+//
+// This file is named "generic" because at a certain point Linux
+// started standardizing on system call numbers across
+// architectures. So far this means only arm64 uses the standard
+// numbers.
 const randomTrap uintptr = 278

src/internal/syscall/unix/getrandom_linux_mips64x.go

@@ -6,4 +6,6 @@
 
 package unix
 
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
 const randomTrap uintptr = 5313

src/internal/syscall/unix/getrandom_linux_ppc64x.go

@@ -6,4 +6,6 @@
 
 package unix
 
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
 const randomTrap uintptr = 359