[spec] Allow explicit keyword definitions (#1553)

Rather than describing keyword tokens as always being defined implicitly by terminal symbols in syntactic productions, describe them as being defined implicitly or explicitly. This accounts for the explicit definitions of `offset` and `align` phrases, which are lexically keywords, later in the chapter.

Fixes #1552.

document/core/text/lexical.rst

@@ -58,7 +58,7 @@ That is, the next token always consists of the longest possible sequence of char
 Tokens can be separated by :ref:`white space <text-space>`,
 but except for strings, they cannot themselves contain whitespace.
 
-The set of *keyword* tokens is defined implicitly, by all occurrences of a :ref:`terminal symbol <text-grammar>` in literal form, such as :math:`\text{keyword}`, in a :ref:`syntactic <text-syntactic>` production of this chapter.
+*Keyword* tokens are defined either implicitly by an occurrence of a :ref:`terminal symbol <text-grammar>` in literal form, such as :math:`\text{keyword}`, in a :ref:`syntactic <text-syntactic>` production of this chapter, or explicitly where they arise in this chapter.
 
 Any token that does not fall into any of the other categories is considered *reserved*, and cannot occur in source text.