doc/go_mem: explain arrays, structs, and complex numbers

Arrays, structs, and complex numbers are collections of values that
are handled separately by the memory model.

An earlier version may have said this, but the current version does not.
Say it.

Change-Id: If3928bed6659e58e688f88aa0dde05423cbb3820
Reviewed-on: https://go-review.googlesource.com/c/go/+/514476
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

doc/go_mem.html

@@ -221,15 +221,26 @@ <h2 id="restrictions">Implementation Restrictions for Programs Containing Data R
 </p>
 
 <p>
-First, any implementation can, upon detecting a data race,
+Any implementation can, upon detecting a data race,
 report the race and halt execution of the program.
 Implementations using ThreadSanitizer
 (accessed with “<code>go</code> <code>build</code> <code>-race</code>”)
 do exactly this.
 </p>
 
 <p>
-Otherwise, a read <i>r</i> of a memory location <i>x</i>
+A read of an array, struct, or complex number
+may by implemented as a read of each individual sub-value
+(array element, struct field, or real/imaginary component),
+in any order.
+Similarly, a write of an array, struct, or complex number
+may be implemented as a write of each individual sub-value,
+in any order.
+</p>
+
+<p>
+A read <i>r</i> of a memory location <i>x</i>
+holding a value
 that is not larger than a machine word must observe
 some write <i>w</i> such that <i>r</i> does not happen before <i>w</i>
 and there is no write <i>w'</i> such that <i>w</i> happens before <i>w'</i>