"Variable Declarations.md" uses the optional parameter operator "?" before it is defined in the file "Functions.md".  Adding a brief explanation so readers new to Typescript can read this documentation in-order without getting confused (as I did).
"Variable Declarations.md" uses the optional parameter operator "?" before it is
defined in the file "Functions.md".  Adding a brief explanation so readers new
to Typescript can read this documentation in-order without getting confused (as
I did).

Microsoft/TypeScript-Handbook