git-reset: expand page (#1127)

As mentioned in https://github.com/tldr-pages/tldr/pull/1088#issuecomment-249368792

pages/common/git-reset.md

@@ -1,11 +1,28 @@
 # git reset
 
 > Undo commits or unstage changes, by resetting the current git HEAD to the specified state.
+> If a path is passed, it works as "unstage"; if a commit hash or branch is passed, it works as "uncommit".
 
-- Undo last commit, keep the changes in the local filesystem:
+- Unstage everything:
 
-`git reset HEAD~1`
+`git reset`
 
-- Undo last commit, permanently delete the changes, and discard staged changes:
+- Unstage specific file(s):
 
-`git reset --hard HEAD~1`
+`git reset {{path/to/file(s)}}`
+
+- Unstage portions of a file:
+
+`git reset -p {{path/to/file}}`
+
+- Undo the last commit, keeping its changes (and any further uncommitted changes) in the filesystem:
+
+`git reset HEAD~`
+
+- Undo the last two commits, adding their changes to the index, i.e. staged for commit:
+
+`git reset --soft HEAD~2`
+
+- Reset the repository to a given commit, discarding committed, staged and uncommitted changes since then:
+
+`git reset --hard {{commit}}`