[DOC] Improve documentation of :include: directive file search

[p8]

The current documentation is incorrect.

1. "The current directory" is searched before the --include directories.

   See:
   

   rdoc/lib/rdoc/markup/pre_process.rb

   Lines 288 to 296 in 4e14158

   	def find_include_file(name)
   	to_search = [File.dirname(@input_file_name)].concat @include_path
   	to_search.each do |dir|
   	full_name = File.join(dir, name)
   	stat = File.stat(full_name) rescue next
   	return full_name if stat.readable?
   	end
   	nil
   	end

2. "Searching the file by default in the current directory" could be read as "searching the file in the directory from which RDoc is run". This is incorrect, as the file is first searched in the directory containing the current documented file.