Update comments and documentation

Author: pradyunsg

src/pip/_internal/resolution/resolvelib/candidates.py

@@ -347,8 +347,8 @@ class ExtrasCandidate(Candidate):
        to treat it as a separate node in the dependency graph.
     2. When we're getting the candidate's dependencies,
        a) We specify that we want the extra dependencies as well.
-       b) We add a dependency on the base candidate (matching the name and
-          version).  See below for why this is needed.
+       b) We add a dependency on the base candidate.
+          See below for why this is needed.
     3. We return None for the underlying InstallRequirement, as the base
        candidate will provide it, and we don't want to end up with duplicates.
 
@@ -417,7 +417,8 @@ def iter_dependencies(self):
                 extra
             )
 
-        # Add a dependency on the exact base.
+        # Add a dependency on the exact base
+        # (See note 2b in the class docstring)
         yield factory.make_requirement_from_candidate(self.base)
 
         for r in self.base.dist.requires(valid_extras):