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
diff --git a/doc/install-source.html b/doc/install-source.html
index ee7eb62..6f6a15a 100644
--- a/doc/install-source.html
+++ b/doc/install-source.html
@@ -457,7 +457,7 @@
</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
