doc/go1.15: enumerate ELF-based OSes

Users don't necessarily know if their OS is ELF-based.

For #37419.

Change-Id: I4a4256c5f5eb34211729e1311582bb0e210f1f8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/238240
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/doc/go1.15.html b/doc/go1.15.html
index df6acc8..33598a2 100644
--- a/doc/go1.15.html
+++ b/doc/go1.15.html
@@ -313,7 +313,8 @@
 <p>
   For a representative set of large Go programs, linking is 20% faster
   and requires 30% less memory on average, for <code>ELF</code>-based
-  OSes running on <code>amd64</code> architectures, with more modest
+  OSes (Linux, FreeBSD, NetBSD, OpenBSD, Dragonfly, and Solaris)
+  running on <code>amd64</code> architectures, with more modest
   improvements for other architecture/OS combinations.
 </p>