docs(@angular/build): note tsconfig fallback for library declarationMap and compilationMode

The library builder no longer sets schema defaults for `declarationMap` and `compilationMode`, so
the values from the TypeScript configuration apply when the options are not set. The option
descriptions did not mention this. Document the fallback, including the `partial` default for
`compilationMode`.

Author: MeAkib

packages/angular/build/src/builders/library/schema.json

@@ -71,11 +71,11 @@
     },
     "declarationMap": {
       "type": "boolean",
-      "description": "Generates a sourcemap for each corresponding '.d.ts' file."
+      "description": "Generates a sourcemap for each corresponding '.d.ts' file. Defaults to the `compilerOptions.declarationMap` value in the TypeScript configuration file."
     },
     "compilationMode": {
       "type": "string",
-      "description": "Angular compilation mode. Use 'partial' when publishing to npm (APF requirement). Use 'full' only during development or for private, internal monorepo packages that are never published.",
+      "description": "Angular compilation mode. Use 'partial' when publishing to npm (APF requirement). Use 'full' only during development or for private, internal monorepo packages that are never published. Defaults to the `angularCompilerOptions.compilationMode` value in the TypeScript configuration file, or 'partial' if not set.",
       "enum": ["partial", "full"]
     },
     "allowedNonPeerDependencies": {