Where is Ruby's ERB format “officially” defined?

Question

A number of sources (e.g. What is the meaning of erb? and three out of the top four results from this google search) cite http://ruby-doc.org/stdlib-1.8.7/libdoc/erb/rdoc/ERB.ht

Answer 1

erb was developed by Masatoshi Seki as a Ruby implementation of eRuby, so its specification almost follows that of eRuby. One difference the author mentions is:

% cat hello.erb
Hello, <% print "World" %>.

% eruby hello.erb
Hello, World.

% erb hello.erb
WorldHello, .

in which case you can do:

% cat hello2.erb
Hello, <%= "World" %>.

% eruby hello2.erb 
Hello, World.

% erb hello2.erb
Hello, World.

to let them work the same way.

Here is an explanation about it by the author, and here and here are documentation written by the author.