Merge branch 'doc/changelog-localindexset-merge' into 'master'

LocalIndexSet has been merged into LocalView a while ago, in commit
ed807839. Somehow it never made it into the
changelog for 2.6, so I propose to add it now and cherry-pick this new
changelog entry to the releases/2.6 branch to be included in case of a 2.6.1
release.

See merge request [!190]

  [!190]: gitlab.dune-project.org/staging/dune-functions/merge_requests/190


(cherry picked from commit ad8bc3e8)

27ac9e58 CHANGELOG.md: document merge of LocalIndexSet and LocalView
e3233eff [doc] Fix typo and add minor detail in changelog