[ci skip] Revise TagHelper only permitting symbols doc.

Follow up to 454460e.

Rebuild the sentence so the bit about only symbols allowed comes less
out of left field and fits in better with the existing doc.

Also remove the `(Legacy syntax)` construct. The sections are properly
nested under `=== Options` with `====` and look fine on the generated
API doc site.

actionview/lib/action_view/helpers/tag_helper.rb

@@ -138,8 +138,7 @@ module ActionView
       #
       # ==== Options
       #
-      # Any passed options become attributes on the generated tag.
-      # You can only use symbols (not strings) for the attribute names.
+      # Use symbol keyed options to add attributes to the generated tag.
       #
       #   tag.section class: %w( kitties puppies )
       #   # => <section class="kitties puppies"></section>
@@ -201,7 +200,7 @@ module ActionView
       # hash to +options+. Set +escape+ to false to disable attribute value
       # escaping.
       #
-      # ==== Options (Legacy syntax)
+      # ==== Options
       #
       # You can use symbols or strings for the attribute names.
       #
@@ -211,7 +210,7 @@ module ActionView
       # HTML5 <tt>data-*</tt> attributes can be set with a single +data+ key
       # pointing to a hash of sub-attributes.
       #
-      # ==== Examples (Legacy syntax)
+      # ==== Examples
       #
       #   tag("br")
       #   # => <br />