doc/go1.15: clarify external linking can still be used for building PIE

In Go 1.15 we switched the default linking mode for PIE on
Linux/AMD64 and Linux/ARM64 to internal linking. Clarify that
the previous behavior (external linking) can still be used with
a flag.

Fixes #40719.

Change-Id: Ib7042622bc91e1b1aa31f520990d03b5eb6c56bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/248199
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.15.html

@@ -357,7 +357,10 @@ <h2 id="linker">Linker</h2>
   The linker now defaults to internal linking mode
   for <code>-buildmode=pie</code> on
   <code>linux/amd64</code> and <code>linux/arm64</code>, so these
-  configurations no longer require a C linker.
+  configurations no longer require a C linker. External linking
+  mode (which was the default in Go 1.14 for
+  <code>-buildmode=pie</code>) can still be requested with
+  <code>-ldflags=-linkmode=external</code> flag.
 </p>
 
 <h2 id="objdump">Objdump</h2>