Specs for single and multi suggestions based on author

Add documentation

changelogs/unreleased/suggestion-author.yml

+---
+title: Commit author for suggestions is note author
+merge_request: 39940
+author:
+type: added

doc/user/discussions/index.md

@@ -515,6 +515,9 @@ to your branch to address your reviewers' requests.
 
    ![A code change suggestion displayed, with the button to apply the batch of suggestions highlighted.](img/apply_batch_of_suggestions_v13_1.jpg "Apply a batch of suggestions")
 
+WARNING:
+Suggestions applied from multiple authors will create a commit authored by the user applying the suggestions.
+
 ## Start a thread by replying to a standard comment
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/30299) in GitLab 11.9

spec/services/suggestions/apply_service_spec.rb

@@ -321,6 +321,73 @@ RSpec.describe Suggestions::ApplyService do
         end
       end
 
+      context 'single suggestion' do
+        let(:author) { suggestions.first.note.author }
+        let(:commit) { project.repository.commit }
+
+        context 'author of suggestion applies suggestion' do
+          before do
+            suggestion.note.update!(author_id: user.id)
+
+            apply(suggestions)
+          end
+
+          it 'created commit by same author and committer' do
+            expect(user.commit_email).to eq(author.commit_email)
+            expect(author).to eq(user)
+            expect(commit.author_email).to eq(author.commit_email)
+            expect(commit.committer_email).to eq(user.commit_email)
+            expect(commit.author_name).to eq(author.name)
+            expect(commit.committer_name).to eq(user.name)
+          end
+        end
+
+        context 'another user applies suggestion' do
+          before do
+            apply(suggestions)
+          end
+
+          it 'created commit has authors info and commiters info' do
+            expect(user.commit_email).not_to eq(user.email)
+            expect(author).not_to eq(user)
+            expect(commit.author_email).to eq(author.commit_email)
+            expect(commit.committer_email).to eq(user.commit_email)
+            expect(commit.author_name).to eq(author.name)
+            expect(commit.committer_name).to eq(user.name)
+          end
+        end
+      end
+
+      context 'multiple suggestions' do
+        let(:author_emails) { suggestions.map {|s| s.note.author.commit_email } }
+        let(:first_author) { suggestion.note.author }
+        let(:commit) { project.repository.commit }
+
+        context 'when all the same author' do
+          before do
+            apply(suggestions)
+          end
+
+          it 'uses first authors information' do
+            expect(author_emails).to include(first_author.commit_email).exactly(3)
+            expect(commit.author_email).to eq(first_author.commit_email)
+          end
+        end
+
+        context 'when all different authors' do
+          before do
+            suggestion2.note.update!(author_id: create(:user).id)
+            suggestion3.note.update!(author_id: create(:user).id)
+            apply(suggestions)
+          end
+
+          it 'uses committers information' do
+            expect(commit.author_email).to eq(user.commit_email)
+            expect(commit.committer_email).to eq(user.commit_email)
+          end
+        end
+      end
+
       context 'multiple suggestions applied sequentially' do
         def apply_suggestion(suggestion)
           suggestion.reload