Tweak `help execution` wording after c0a46c5.

This improves the wording a bit more to be more precise.

Change-Id: I50f9f907999e4430bdda118ca36215dfe72c5b0c
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/18520
Reviewed-by: David Turner <digit@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>

docs/reference.md

diff --git a/docs/reference.md b/docs/reference.md
index 88007f5..36ccd6e 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -7380,8 +7380,8 @@
      necessary to resolve dependencies. If a BUILD file isn't found in the
      specified location, GN will look in the corresponding location inside
      the secondary_source defined in the dotfile (see "gn help dotfile").
-     Each BUILD.gn file will again be executed in a new scope containing
-     only the definitions from the BUILDCONFIG.gn's scope.
+     Each BUILD.gn file will again be executed in a new scope whose only
+     parent is BUILDCONFIG.gn's scope.
 
   6. When a target's dependencies are resolved, write out the `.ninja`
      file to disk.

src/gn/target.cc

diff --git a/src/gn/target.cc b/src/gn/target.cc
index 8a14ff1..11063fb 100644
--- a/src/gn/target.cc
+++ b/src/gn/target.cc
@@ -233,8 +233,8 @@
      necessary to resolve dependencies. If a BUILD file isn't found in the
      specified location, GN will look in the corresponding location inside
      the secondary_source defined in the dotfile (see "gn help dotfile").
-     Each BUILD.gn file will again be executed in a new scope containing
-     only the definitions from the BUILDCONFIG.gn's scope.
+     Each BUILD.gn file will again be executed in a new scope whose only
+     parent is BUILDCONFIG.gn's scope.
 
   6. When a target's dependencies are resolved, write out the `.ninja`
      file to disk.