Add documentation about no sources for GN guide.

tools/gn/docs/style_guide.md

Index: tools/gn/docs/style_guide.md
diff --git a/tools/gn/docs/style_guide.md b/tools/gn/docs/style_guide.md
index 29d6b31e418fd81f92f0c19c7ac1263287c0264c..a87ad8f6a53720da07632145a7f1dea66759cd94 100644
--- a/tools/gn/docs/style_guide.md
+++ b/tools/gn/docs/style_guide.md
@@ -183,6 +183,11 @@ to the link line of the final binary.
     and static libraries depending on the build configuration to avoid this
     problem.
 
+  * Source sets can have no sources, while static libraries will give strange
+    platform-specific errors if they have no sources. If a target has only
+    headers (for include checking purposes) or conditionally has no sources on
+    sone platforms, use a source set.
+
   * In cases where a lot of the symbols are not needed for a particular link
     (this especially happens when linking test binaries), putting that code in
     a static library can dramatically increase linking performance. This is