writing_gn_templates.md: Clarify *why* __suffix is beneficial

TBR=minor doc update

Change-Id: I8d02a5b686009d45ba5fa9033f221157247b0101
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2541910Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827888}

build/docs/writing_gn_templates.md

@@ -175,6 +175,9 @@ template("my_template") {
 }
 ```
 
+This scheme ensures that subtargets defined in templates do not conflict with
+top-level targets.
+
 ### Variables
 Prefix variables within templates and targets with an underscore. For example: