config: add hardlink_lock description

Fixes iterative#858

static/docs/command-reference/config.md

@@ -55,7 +55,7 @@ corresponding config file.
 
 ## Configuration sections
 
-The following config sections are written by this command to the project config 
+The following config sections are written by this command to the project config
 file (in `.dvc/config` by default), and they support the options below:
 
 ### core
@@ -79,6 +79,11 @@ This is the main section with the general config options:
 - `core.checksum_jobs` - number of threads for computing checksums. Accepts
   positive integers. The default value is `max(1, min(4, cpu_count() // 2))`.
 
+- `core.hardlink_lock` - use `hardlink`-based locks instead of `flock`-based
+  ones (e.g. repository lock .dvc/lock). Useful when you have your DVC
+  repository on a filesystem that doesn't properly support file locking (e.g.
+  [NFS v3 and older](http://nfs.sourceforge.net/)).
+
 ### remote
 
 These are sections in the config file that describe particular remotes. These
@@ -169,8 +174,8 @@ for more details.) This section contains the following options:
 
 ### state
 
-See [DVC Files and Directories](/doc/user-guide/dvc-files-and-directories) to learn
-more about the state file (database) that is used for optimization.
+See [DVC Files and Directories](/doc/user-guide/dvc-files-and-directories) to
+learn more about the state file (database) that is used for optimization.
 
 - `state.row_limit` - maximum number of entries in the state database, which
   affects the physical size of the state file itself, as well as the performance