Fix #5325 : added --keep flag to dvc exp remove documentation (#5326)

* added --keep flag to dvc exp remove

* Fixed doc for --keep as mentioned in #5326 (comment)

* Added examples

* prettier code style - formatting issues fixed

---------

Co-authored-by: Ivan Shcheklein <[email protected]>

content/docs/command-reference/exp/remove.md

@@ -8,7 +8,7 @@ Delete specific experiments from the <abbr>project</abbr>.
 
 ```usage
 usage: dvc exp remove [-h] [-q | -v] [-A] [--rev <commit>] [-n <num>]
-                      [--queue | -g <git_remote>]
+                      [--queue | -g <git_remote>] [--keep]
                       [<name> [<name> ...]]
 
 positional arguments:
@@ -40,6 +40,10 @@ With `--queue`, the list of experiments awaiting execution is cleared instead.
 - `-g`, `--git-remote` - Name or URL of the Git remote to remove the experiment
   from
 
+- `--keep` - changes the default behavior to the opposite for options like `-n`,
+  `--rev`. For example `-n <num> --keep` keeps experiments from the last `num`
+  commits and removes all other experiments.
+
 - `-h`, `--help` - shows the help message and exit.
 
 - `-q`, `--quiet` - do not write anything to standard output. Exit with 0 if no
@@ -121,3 +125,25 @@ $ dvc exp list myremote
 master:
         conic-ease
 ```
+
+Conversely, to keep only specific experiments (and remove all others), give
+their names to `dvc exp remove` with the `--keep` flag :
+
+```cli
+$ dvc exp list
+master:
+        major-mela
+        conic-ease
+        lucid-lair
+
+$ dvc exp remove --keep major-mela
+
+$ dvc exp list
+master:
+        major-mela
+
+```
+
+In this case, the experiments named `conic-ease` and `lucid-lair` are removed,
+and only `major-mela` is kept. The `--keep` flag also works with `--num` (`-n`)
+and `--rev`, but _not_ with `--queue`.