Update Linux build docs to remove GYP references.

This removes most references to the GYP build from files in docs starting with "linux_".

Review URL: https://codereview.chromium.org/1877013002

Cr-Commit-Position: refs/heads/master@{#386526}
diff --git a/docs/linux_suid_sandbox_development.md b/docs/linux_suid_sandbox_development.md
index 4563a31..03aceca 100644
--- a/docs/linux_suid_sandbox_development.md
+++ b/docs/linux_suid_sandbox_development.md
@@ -1,5 +1,9 @@
 # Linux SUID Sandbox Development
 
+*IMPORTANT NOTE: The Linux SUID sandbox is almost but not completely removed.
+See https://bugs.chromium.org/p/chromium/issues/detail?id=598454
+This page is mostly out-of-date.*
+
 For context see [LinuxSUIDSandbox](linux_suid_sandbox.md)
 
 We need a SUID helper binary to turn on the sandbox on Linux.