Doc most DSL methods as write-only

The public readers for these attributes are on the instance, not the
class.

Library/Homebrew/formula.rb

@@ -777,7 +777,7 @@ class Formula
     # @private
     attr_reader :keg_only_reason
 
-    # @!attribute [rw]
+    # @!attribute [w]
     # The homepage for the software. Used by users to get more information
     # about the software and Homebrew maintainers as a point of contact for
     # e.g. submitting patches.
@@ -792,7 +792,7 @@ class Formula
     # @private
     attr_reader :plist_manual
 
-    # @!attribute [rw]
+    # @!attribute [w]
     # Used for creating new Homebrew versions of software without new upstream
     # versions. For example, if we bump the major version of a library this
     # {Formula} {.depends_on} then we may need to update the `revision` of this
@@ -806,7 +806,7 @@ class Formula
       @specs ||= [stable, devel, head].freeze
     end
 
-    # @!attribute [rw] url
+    # @!attribute [w] url
     # The URL used to download the source for the {#stable} version of the formula.
     # We prefer `https` for security and proxy reasons.
     def url val, specs={}
@@ -821,7 +821,7 @@ class Formula
       stable.version(val)
     end
 
-    # @!attribute [rw] mirror
+    # @!attribute [w] mirror
     # Additional URLs for the {#stable} version of the formula.
     # These are only used if the {.url} fails to download. It's optional and
     # there can be more than one. Generally we add them when the main {.url}
@@ -831,14 +831,14 @@ class Formula
       stable.mirror(val)
     end
 
-    # @!attribute [rw] sha1
+    # @!attribute [w] sha1
     # @scope class
     # To verify the {#cached_download}'s integrity and security we verify the
     # SHA-1 hash matches what we've declared in the {Formula}. To quickly fill
     # this value you can leave it blank and run `brew fetch --force` and it'll
     # tell you the currently valid value.
 
-    # @!attribute [rw] sha256
+    # @!attribute [w] sha256
     # @scope class
     # Similar to {.sha1} but using a SHA-256 hash instead.