Improve CSS for "toggle source" hovering over one more method signatures

[ybiquitous]

This PR improves the behavior of showing the "toggle source" element on mouseover.

For example, when a method has one more signatures by using :call-seq:,

# :call-seq:
#   foo {|element| ... } -> self
#   foo -> new_enumeration
def foo
end

The current CSS doesn't show "toggle source" even when hovering the second signature foo -> new_enumeration.
But this change will show "toggle source" always when hovering over any signature.

For details about the .method-header element, see lib/rdoc/generator/template/darkfish/class.rhtml:

rdoc/lib/rdoc/generator/template/darkfish/class.rhtml

Lines 101 to 124 in 0e060c6

	<div class="method-header">
	<%- if (call_seq = method.call_seq) then -%>
	<%- call_seq.strip.split("\n").each_with_index do |call_seq, i| -%>
	<div class="method-heading">
	<span class="method-callseq">
	<%= h(call_seq.strip.
	gsub( /^\w+\./m, '')).
	gsub(/(.*)[-=]&gt;/, '\1&rarr;') %>
	</span>
	<%- if i == 0 and method.token_stream then -%>
	<span class="method-click-advice">click to toggle source</span>
	<%- end -%>
	</div>
	<%- end -%>
	<%- else -%>
	<div class="method-heading">
	<span class="method-name"><%= h method.name %></span><span
	class="method-args"><%= h method.param_seq %></span>
	<%- if method.token_stream then -%>
	<span class="method-click-advice">click to toggle source</span>
	<%- end -%>
	</div>
	<%- end -%>
	</div>

For example, see https://docs.ruby-lang.org/en/3.2/Array.html#method-i-delete

Test locally

1. Add :call-seq: temporarily like this:

--- a/lib/rdoc.rb
+++ b/lib/rdoc.rb
@@ -198,4 +198,9 @@ module RDoc
   autoload :Extend,         "#{__dir__}/rdoc/extend"
   autoload :Require,        "#{__dir__}/rdoc/require"
 
+  # :call-seq:
+  #   foo {|element| ... } -> self
+  #   foo -> new_enumeration
+  def foo
+  end
 end

2. Run bundle exec rake rerdoc to build
3. Run open _site/RDoc.html
   •