[release-branch.go1.3] doc/install-source.html: document that GO386 will be auto-detected when building on both 386 and amd64.
[release-branch.go1.3] doc/install-source.html: document that GO386 will be auto-detected when building on both 386 and amd64.

««« CL 102150046 / ccf7893cc2f0
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
»»»

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/105830044

klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros