document locking in dvc run/repro

Fixes iterative#860

static/docs/command-reference/repro.md

@@ -44,6 +44,10 @@ files, intermediate or final results. It saves all the data files, intermediate
 or final results into the <abbr>DVC cache</abbr> (unless `--no-commit` option is
 specified), and updates stage files with the new checksum information.
 
+### Locking
+
+See [Locking](/doc/command-reference/run#Locking).
+
 ## Options
 
 - `-f`, `--force` - reproduce a pipeline, regenerating its results, even if no

static/docs/command-reference/run.md

@@ -72,6 +72,14 @@ must produce the same results given the same inputs/dependencies).
 Have in mind what brings entropy to your command (e.g. random generators, time,
 hardware, etc.) and try to minize it (e.g. fix seeds).
 
+### Locking
+
+When running your command, DVC will release its repository lock (.dvc/lock), so
+that you will be able to run other DVC commands in parallel. However, it uses
+per-path read-write locking instead, to guarantee that no two DVC instances
+would be writing to the same path and don't write to paths that are being read
+from by another instance.
+
 ## Options
 
 - `-d`, `--deps` - specify a file or a directory the stage depends on. Multiple