💹 Update GN docs about target_cpu to encourage use of current_cpu

tools/gn/docs/cross_compiles.md

Index: tools/gn/docs/cross_compiles.md
diff --git a/tools/gn/docs/cross_compiles.md b/tools/gn/docs/cross_compiles.md
index 68d9bcb8570a3b623bb35ad0b1ecc2f426a64807..36f01db7523b2e0c62e10a59c1ae93527d423493 100644
--- a/tools/gn/docs/cross_compiles.md
+++ b/tools/gn/docs/cross_compiles.md
@@ -73,11 +73,42 @@ which target\_arch is requested; the values are constant across all
 toolchains. You can do similar things for the `host_cpu` and `host_os`
 variables, but should generally never need to.
 
+For the default toolchain, `target_cpu` and `current_cpu` are the same. For a
+secondary toolchain, `current_cpu` is set based on the toolchain definition
+and `target_cpu` remains the same. When writing rules, **`current_cpu` should
+be used rather than `target_cpu` most of the time**.
+
 By default, dependencies listed in the `deps` variable of a rule use the
 same (currently active) toolchain. You may specify a different toolchain
 using the `foo(bar)` label notation as described in
 [GNLanguage#Labels](language.md#Labels).
 
+Here's an example of when to use `target_cpu` vs `current_cpu`:
+
+```
+config("my_config") {
+  # Uses current_cpu because compile flags are toolchain-dependent.
+  if (current_cpu == "arm") {
+    defines = [ "CPU_IS_32_BIT" ]
+  } else {
+    defines = [ "CPU_IS_64_BIT" ]
+  }
+}
+
+group("fat_bundle") {

[agrieve, 2016/06/08 17:43:56]

hmm, actually, I don't think this example is correct.

E.g., if target_cpu == "arm", and a target explicitly build
":fat_bundle(//build/toolchain/android:arm64)", it would do the wrong thing.

Only valid example I could find in our code was from here:
https://cs.chromium.org/chromium/src/build/config/sysroot.gni

So changed to this.

+  deps = [
+    ":shared_library",
+  ]
+  # Uses target_cpu because 64 bit devices require both 64 bit and 32 bit
+  # libraries, but 32 bit devices require only 32 bit libraries.
+  if (target_cpu == "arm64") {
+    deps += [
+      ":shared_library(//build/toolchain/android:arm)",
+    ]
+  }
+}
+```
+
 ## As a //build/config or //build/toolchain author
 
 As described in