Add description of string to language reference

Ref. #292, #704

doc/Language-Reference.adoc

@@ -5,10 +5,13 @@
 The specification language describes protocol message formats based on types. For each type of the specification language a description of its syntax and semantics and an example is given. A simple variant of Backus-Naur Form is used to describe the syntax. Reserved keywords and literals are marked in *bold*. References to syntactic categories are xref:none[highlighted]. To convey some semantic information the name of some syntactic categories are prefixed by a non-highlighted part. Syntactic categories with prefixes are equivalent to the category without the prefix. The following basic elements are used to describe the syntax of the language:
 
 [[syntax-name]]
-name: A name consists of alphanumeric characters and underscores. By convention a name starts with a capital and after each underscore follows a capital as well (e.g., Mixed_Case_With_Underscores).
+name: A name consists of alphanumeric characters and underscores. By convention a name starts with a capital and after each underscore follows a capital as well (e.g., `Mixed_Case_With_Underscores`).
 
 [[syntax-number]]
-number: A number consists of numerical digits. An underscore can be added between two digits to improve readability.
+number: A number consists of numerical digits. An underscore can be added between two digits to improve readability (e.g., `1_234`).
+
+[[syntax-string]]
+string: A string literal is a sequence of characters delimited by double quotes (e.g., `"String"`).
 
 [[syntax-mathematical_expression]]
 mathematical_expression: A mathematical expression consists of numbers and names combined by mathematical operators (addition *+*, subtraction *-*, multiplication *\**, division */*, exponentiation **********).