doc/install-source.html: document that GO386 will be auto-detected when building…

doc/install-source.html: document that GO386 will be auto-detected when building on both 386 and amd64.
Fixes #8152.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/102150046

doc/install-source.html

@@ -457,7 +457,7 @@ installs all commands there.
 </p>
 
 <li><code>$GO386</code> (for <code>386</code> only, default is auto-detected
-if built natively, <code>387</code> if not)
+if built on either <code>386</code> or <code>amd64</code>, <code>387</code> otherwise)
 <p>
 This controls the code generated by 8g to use either the 387 floating-point unit
 (set to <code>387</code>) or SSE2 instructions (set to <code>sse2</code>) for