Submodule GitExtensionsDoc updated

Submodule GitExtensionsDoc:
    > a120c4e - Fix incomplete sentence.
move user manual handling out of UI
move open in default browser functionality out of UI
Merge branch 'pr/n1918_mexx'
Corrected URIs
Removed compiler warnings.
DeleteUnusedBranchesForm: add Settings button to directly go to corresponding plugin settings page
DeleteUnusedBranchesForm: set minimum size for dialog
BackgroundFetchPlugin: if invoked via menu show SettingsDialog instead of doing nothing
rearrange layout
RevisionGridMenuCommands: fix: "Show remote branch" menu item checkbox did not change checkmark when RevisionGrid is part of Choose Commit dialog
Added likn to wiki page
"Show revision graph" as MenuCommand
"Draw non relatives gray" as MenuCommand
"Order revisions by date" and "Show author date" as MenuCommand
"Show relative date", "Show git notes" and "Show tags" as MenuCommand
remove Show*Branches items from designer
Fill RevisionGrid View context from MenuCommands
Fill RevisionGrid Navigate context from MenuCommands
Fix #1660: replace GitEx with GitExt