Document cc_toolchain to avoid unnecessary builds

Document that tags = ["manual"] should be used.

Fixes #10641

RELNOTES:none
PiperOrigin-RevId: 311483431

src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainRule.java

diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainRule.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainRule.java
index 1452a37..4839253 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainRule.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainRule.java
@@ -400,4 +400,8 @@
     page
   </a> for elaborate C++ toolchain configuration and toolchain selection documentation.
 </p>
+<p>
+  Use <code>tags = ["manual"]</code> in order to prevent toolchains from being built and configured
+  unnecessarily when invoking <code>bazel build //...</code>
+</p>
 <!-- #END_BLAZE_RULE -->*/