add createProgram docs (#15697)

* add `createProgram` docs

* updated `createProgram` docs

Author: donaldpipowitch

src/compiler/program.ts

@@ -386,6 +386,19 @@ namespace ts {
         allDiagnostics?: Diagnostic[];
     }
 
+    /**
+     * Create a new 'Program' instance. A Program is an immutable collection of 'SourceFile's and a 'CompilerOptions'
+     * that represent a compilation unit.
+     *
+     * Creating a program proceeds from a set of root files, expanding the set of inputs by following imports and
+     * triple-slash-reference-path directives transitively. '@types' and triple-slash-reference-types are also pulled in.
+     *
+     * @param rootNames - A set of root files.
+     * @param options - The compiler options which should be used.
+     * @param host - The host interacts with the underlying file system.
+     * @param oldProgram - Reuses an old program structure.
+     * @returns A 'Program' object.
+     */
     export function createProgram(rootNames: string[], options: CompilerOptions, host?: CompilerHost, oldProgram?: Program): Program {
         let program: Program;
         let files: SourceFile[] = [];