Improve documentation of ActiveSupport::TimeZone.create [ci-skip]

The `create` method is currently marked as an alias of `new`. However,
because `new` is later overridden, it's no longer an alias.

This requires wrapping the `alias_method` with stopdoc/startdoc, as the
method is still marked as an alias otherwise (adding `:nodoc:` doesn't
help).

The `initialize` method has to be wrapped with stopdoc/startdoc as well,
as the `new` method will still be documented for the initialized.
Adding a `:nodoc:` instead will remove documentation for all following
methods.

activesupport/lib/active_support/values/time_zone.rb

@@ -208,7 +208,9 @@ def find_tzinfo(name)
         TZInfo::Timezone.get(MAPPING[name] || name)
       end
 
+      # :stopdoc:
       alias_method :create, :new
+      # :startdoc:
 
       # Returns a TimeZone instance with the given name, or +nil+ if no
       # such TimeZone instance exists. (This exists to support the use of
@@ -296,15 +298,22 @@ def zones_map
     attr_reader :name
     attr_reader :tzinfo
 
+    ##
+    # :singleton-method: create
+    # :call-seq: create(name, utc_offset = nil, tzinfo = nil)
+    #
     # Create a new TimeZone object with the given name and offset. The
     # offset is the number of seconds that this time zone is offset from UTC
     # (GMT). Seconds were chosen as the offset unit because that is the unit
     # that Ruby uses to represent time zone offsets (see Time#utc_offset).
+
+    # :stopdoc:
     def initialize(name, utc_offset = nil, tzinfo = nil)
       @name = name
       @utc_offset = utc_offset
       @tzinfo = tzinfo || TimeZone.find_tzinfo(name)
     end
+    # :startdoc:
 
     # Returns the offset of this time zone from UTC in seconds.
     def utc_offset