Fix typos in changelog and release notes

Author: sequba

CHANGELOG.md

@@ -16,9 +16,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ### Changed
 
-- Improve efficiency of the default date/time parsing
+- Improved the efficiency of the default date/time parsing
   methods. [#876](https://github.com/handsontable/hyperformula/issues/876)
-- Improve efficiency of the operations on the dependency
+- Improved the efficiency of the operations on the dependency
   graph. [#876](https://github.com/handsontable/hyperformula/issues/876)
 
 ### Fixed

docs/guide/release-notes.md

@@ -17,9 +17,9 @@ HyperFormula adheres to
 
 ### Changed
 
-- Improve efficiency of the default date/time parsing
+- Improved the efficiency of the default date/time parsing
   methods. [#876](https://github.com/handsontable/hyperformula/issues/876)
-- Improve efficiency of the operations on the dependency
+- Improved the efficiency of the operations on the dependency
   graph. [#876](https://github.com/handsontable/hyperformula/issues/876)
 
 ### Fixed

src/interpreter/InterpreterValue.ts

@@ -6,7 +6,11 @@
 import {CellError} from '../Cell'
 import {SimpleRangeValue} from '../SimpleRangeValue'
 
+/**
+ * A symbol representing an empty cell value.
+ */
 export const EmptyValue = Symbol('Empty value')
+
 export type EmptyValueType = typeof EmptyValue
 export type InternalNoErrorScalarValue = RichNumber | RawNoErrorScalarValue
 export type InternalScalarValue = RichNumber | RawScalarValue