bash-completion: add caveat for brew bash and Linux users

Formula/b/bash-completion.rb

@@ -33,8 +33,7 @@ class BashCompletion < Formula
     conflicts_with "util-linux", because: "both install `mount`, `rfkill`, and `rtcwake` completions"
   end
 
-  conflicts_with "bash-completion@2",
-    because: "each are different versions of the same formula"
+  conflicts_with "bash-completion@2", because: "each are different versions of the same formula"
   conflicts_with "medusa", because: "both install `medusa` bash completion"
 
   # Backports the following upstream patch from 2.x:
@@ -58,10 +57,23 @@ def install
   end
 
   def caveats
-    <<~EOS
+    s = <<~EOS
       Add the following line to your ~/.bash_profile:
         [[ -r "#{etc}/profile.d/bash_completion.sh" ]] && . "#{etc}/profile.d/bash_completion.sh"
     EOS
+    version_caveat = <<~EOS
+
+      This formula is mainly for use with Bash 3. If you are using Homebrew's Bash or your
+      system Bash is at least version 4.2, then you should install `bash-completion@2` instead.
+    EOS
+    if Formula["bash"].any_version_installed?
+      s += version_caveat
+    else
+      on_linux do
+        s += version_caveat
+      end
+    end
+    s
   end
 
   test do