doc: mention the use of replacements to resolve imports for 1.12

Updates #26241

Change-Id: I8ffac13d9cc1ee4d4de8fcd2042a7fa60fca567b
Reviewed-on: https://go-review.googlesource.com/c/153157
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.12.html

@@ -138,6 +138,17 @@ <h3 id="modules">Modules</h3>
   that build fails.
 </p>
 
+<p><!-- CL 152739 -->
+  When an import cannot be resolved using the active modules,
+  the <code>go</code> command will now try to use the modules mentioned in the
+  main module's <code>replace</code> directives before consulting the module
+  cache and the usual network sources.
+  If a matching replacement is found but the <code>replace</code> directive does
+  not specify a version, the <code>go</code> command uses a pseudo-version
+  derived from the zero <code>time.Time</code> (such
+  as <code>v0.0.0-00010101000000-000000000000</code>).
+</p>
+
 <h3 id="compiler">Compiler toolchain</h3>
 
 <p><!-- CL 134155, 134156 -->