docs: mention the different types of exact specifications and pep440

Author: KotlinIsland

docs/dependency-specification.md

@@ -79,11 +79,15 @@ Multiple version requirements can also be separated with a comma, e.g. `>= 1.2,
 
 You can specify the exact version of a package.
 
-`==1.2.3` is an example of an exact version specification.
+`1.2.3` is an example of an exact version specification.
 
 This will tell Poetry to install this version and this version only.
 If other dependencies require a different version, the solver will ultimately fail and abort any install or update procedures.
 
+Exact versions can also be specified with `==` according to [PEP 440](https://peps.python.org/pep-0440/).
+
+`==1.2.3` is an example of this.
+
 ### Using the `@` operator
 
 When adding dependencies via `poetry add`, you can use the `@` operator.