Add YARD docs to Faker::String (#2004)

* Add YARD docs to Faker::String

* Disable rubocop to show UTF-8 characters in @examples

lib/faker/default/string.rb

@@ -3,6 +3,22 @@
 module Faker
   class String < Base
     class << self
+      ##
+      # Produces a random UTF-8 string with optional nested length selectors.
+      #
+      # @param length [Integer, Range, Array<Integer, Range, nil>] The length of produced string and/or specific UTF-8 characters to use.
+      # @return [String]
+      #
+      # @example
+      #   rubocop:disable Style/AsciiComments
+      #   Faker::String.random #=> "3 뇦\u0017&y\u{3A109}$8^4* 녹豿4좘툢ꔾ쉙6ɉ\uA6 8TN畀챵|\"3쇤Ŵ"
+      #   Faker::String.random(length: 4) #=> "⼨%0*"
+      #   Faker::String.random(length: 3..12) #=> "\u{69FDC};秨툫"
+      #   Faker::String.random(length: [0, 6]) #=> "I轤𣴒P溟L"
+      #   Faker::String.random(length: [1, (2..5), [3, 6], nil]) #=> "葓L#ћ"
+      #   rubocop:enable Style/AsciiComments
+      #
+      # @faker.version 1.9.0
       def random(legacy_length = NOT_GIVEN, length: 32)
         warn_for_deprecated_arguments do |keywords|
           keywords << :length if legacy_length != NOT_GIVEN
@@ -38,9 +54,9 @@ def char_space_ratio
 
       def utf8character
         sample([
-                 rand(32..90), # latin alphabet
-                 rand(128),             # 7-bit ASCII
-                 rand(0xd800),          # utf-8 codepoints below utf-16 surrogate halves
+                 rand(32..90),           # latin alphabet
+                 rand(128),              # 7-bit ASCII
+                 rand(0xd800),           # utf-8 codepoints below utf-16 surrogate halves
                  rand(57_344..1_114_111) # utf-8 codepoints above utf-16 surrogate halves
                ]).chr(Encoding::UTF_8)
       end