docs: Update tap-pin and tap-unpin disable explanations

I interpreted the existing message as meaning "you don't pin
a tap any more, rather you pin a specific formula from that
tap". I.e. the command still worked, but it had to be done
on a per-formula basis (eg. `brew tap-pin linuxbrew/xorg/mesa` instead
of just `brew tap-pin linuxbrew/xorg`)

IMO, this makes it clearer that the command itself is no longer
supported.

Library/Homebrew/cmd/tap-pin.rb

@@ -12,7 +12,7 @@ module Homebrew
   end
 
   def tap_pin
-    odisabled "brew tap-pin user/tap",
+    odisabled "the brew tap-pin command",
-              "fully-scoped user/tap/formula naming"
+              "fully-scoped user/tap/formula naming when installing and in dependency references"
   end
 end

Library/Homebrew/cmd/tap-unpin.rb

@@ -12,7 +12,7 @@ module Homebrew
   end
 
   def tap_unpin
-    odisabled "brew tap-pin user/tap",
+    odisabled "the brew tap-unpin command",
-              "fully-scoped user/tap/formula naming"
+              "fully-scoped user/tap/formula naming when installing and in dependency references"
   end
 end

Library/Homebrew/formulary.rb

@@ -491,8 +491,8 @@ module Formulary
     if possible_pinned_tap_formulae.size == 1
       selected_formula = factory(possible_pinned_tap_formulae.first, spec)
       if core_path(ref).file?
-        odisabled "brew tap-pin user/tap",
+        odisabled "the brew tap-pin command",
-                  "fully-scoped user/tap/formula naming"
+                  "fully-scoped user/tap/formula naming when installing and in dependency references"
       end
       selected_formula
     else