Clarify before-fetch/after-fetch hook docstrings

The hook receives a Bundler spec proxy (Bundler::EndpointSpecification
or Bundler::RemoteSpecification) that responds to the Gem::Specification
API but is not itself a Gem::Specification instance. Plugins doing
strict is_a? checks would break on the previous wording.

Also clarify the cache-hit language: the hook does not fire when the
initial download-cache check in fetch_gem finds the .gem already on
disk, but Bundler.rubygems.download_gem also has a race-protection
early return on the same path, which the previous wording obscured.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

bundler/lib/bundler/plugin/events.rb

@@ -50,13 +50,19 @@ def self.defined_event?(event)
 
       # @!parse
       #   A hook called before each individual gem is downloaded from a remote source.
-      #   Includes a Gem::Specification. Does not fire on cache hits.
+      #   Includes a spec-like object responding to the Gem::Specification API
+      #   (for example, a Bundler spec proxy such as Bundler::EndpointSpecification
+      #   or Bundler::RemoteSpecification). Does not fire when the gem is already
+      #   present at the initial download-cache check.
       #   GEM_BEFORE_FETCH = "before-fetch"
       define :GEM_BEFORE_FETCH, "before-fetch"
 
       # @!parse
       #   A hook called after each individual gem is downloaded from a remote source.
-      #   Includes a Gem::Specification. Does not fire on cache hits.
+      #   Includes a spec-like object responding to the Gem::Specification API
+      #   (for example, a Bundler spec proxy such as Bundler::EndpointSpecification
+      #   or Bundler::RemoteSpecification). Does not fire when the gem is already
+      #   present at the initial download-cache check.
       #   GEM_AFTER_FETCH = "after-fetch"
       define :GEM_AFTER_FETCH, "after-fetch"