doc: clarify duplicate symbol condition in cgo

Spell out what will happen if a declaration and definition is included
in the same file, should help people who run into duplicate symbol
errors and search for relevant keywords.

This edit is based on opening issue #11263 erroneously.

Change-Id: I0645a9433b8668d2ede9b9a3f6550d802c26388b
Reviewed-on: https://go-review.googlesource.com/11247Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/doc.go

@@ -207,8 +207,10 @@ Not all Go types can be mapped to C types in a useful way.
 
 Using //export in a file places a restriction on the preamble:
 since it is copied into two different C output files, it must not
-contain any definitions, only declarations. Definitions must be
-placed in preambles in other files, or in C source files.
+contain any definitions, only declarations. If a file contains both
+definitions and declarations, then the two output files will produce
+duplicate symbols and the linker will fail. To avoid this, definitions
+must be placed in preambles in other files, or in C source files.
 
 Using cgo directly