Update BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE documentation

With trunk stable, we no longer have multiple development branches, and
this flag is no longer in use.

Test: N/A (documentation change)
Bug: 308188056
Change-Id: Icdaddde9d107de9a453106cae35645b7e6cc21e9
diff --git a/core/product.mk b/core/product.mk
index f8634ef..0a761fb 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -418,8 +418,9 @@
 # /system/etc/security/fsverity/BuildManifest.apk
 _product_single_value_vars += PRODUCT_FSVERITY_GENERATE_METADATA
 
-# If true, sets the default for MODULE_BUILD_FROM_SOURCE. This overrides
-# BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE but not an explicitly set value.
+# If true, this builds the mainline modules from source. This overrides any
+# prebuilts selected via RELEASE_APEX_CONTRIBUTIONS_* build flags for the
+# current release config.
 _product_single_value_vars += PRODUCT_MODULE_BUILD_FROM_SOURCE
 
 # If true, installs a full version of com.android.virt APEX.
diff --git a/target/product/module_common.mk b/target/product/module_common.mk
index bf146a0..da4ea23 100644
--- a/target/product/module_common.mk
+++ b/target/product/module_common.mk
@@ -24,8 +24,9 @@
 # uses -DENFORCE_VINTF_MANIFEST. See b/185759877
 PRODUCT_SHIPPING_API_LEVEL := 29
 
-# Builds using a module product should build modules from source, even if
-# BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE says otherwise.
+# If true, this builds the mainline modules from source. This overrides any
+# prebuilts selected via RELEASE_APEX_CONTRIBUTIONS_* build flags for the
+# current release config.
 PRODUCT_MODULE_BUILD_FROM_SOURCE := true
 
 # Build sdk from source if the branch is not using slim manifests.