💹 Update GN docs about target_cpu to encourage use of current_cpu

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index 0eb5aabcf62448438bda02ca2c06cc559e5a95e6..430db545c721635021ede8968824b735f9d03e4c 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -52,13 +52,13 @@ const char kTargetCpu_Help[] =
     "\n"
     "  This value should be used to indicate the desired architecture for\n"
     "  the primary objects of the build. It will match the cpu architecture\n"
-    "  of the default toolchain.\n"
+    "  of the default toolchain, but not necessarily the current toolchain.\n"
     "\n"
     "  In many cases, this is the same as \"host_cpu\", but in the case\n"
-    "  of cross-compiles, this can be set to something different. This \n"
-    "  value is different from \"current_cpu\" in that it can be referenced\n"
-    "  from inside any toolchain. This value can also be ignored if it is\n"
-    "  not needed or meaningful for a project.\n"
+    "  of cross-compiles, this can be set to something different. This\n"
+    "  value is different from \"current_cpu\" in that it does not change\n"
+    "  based on the current toolchain. When writing rules, current_cpu is\n"
+    "  usually the correct variable to use rather than target_cpu.\n"
     "\n"
     "  This value is not used internally by GN for any purpose, so it\n"
     "  may be set to whatever value is needed for the build.\n"