Document whitespace in quoted dependency specifiers

Author: Herrtian

source/specifications/dependency-specifiers.rst

@@ -44,6 +44,9 @@ include extras, which expand the dependencies of the named distribution to
 enable optional features. The version installed can be controlled using
 version limits, or giving the URL to a specific artifact to install. Finally
 the dependency can be made conditional using environment markers.
+When a dependency specifier is written as a quoted string, authors may keep
+optional whitespace for readability, such as around version specifiers and
+version clauses.
 
 Grammar
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