docs: mention trace2 target-dir mode in git-config

Move the description of trace2's target-directory behavior into the
shared trace2-target-values file so that it is included in both the
git-config and api-trace2 docs. Leave the SID discussion only in
api-trace2 since it's a technical detail.

Signed-off-by: Josh Steadmon <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: steadmon

Documentation/technical/api-trace2.txt

@@ -142,10 +142,9 @@ system or global config value to one of the following:
 
 include::../trace2-target-values.txt[]
 
-If the target already exists and is a directory, the traces will be
-written to files (one per process) underneath the given directory. They
-will be named according to the last component of the SID (optionally
-followed by a counter to avoid filename collisions).
+When trace files are written to a target directory, they will be named according
+to the last component of the SID (optionally followed by a counter to avoid
+filename collisions).
 
 == Trace2 API
 

Documentation/trace2-target-values.txt

@@ -2,7 +2,9 @@
 * `0` or `false` - Disables the target.
 * `1` or `true` - Writes to `STDERR`.
 * `[2-9]` - Writes to the already opened file descriptor.
-* `<absolute-pathname>` - Writes to the file in append mode.
+* `<absolute-pathname>` - Writes to the file in append mode. If the target
+already exists and is a directory, the traces will be written to files (one
+per process) underneath the given directory.
 * `af_unix:[<socket_type>:]<absolute-pathname>` - Write to a
 Unix DomainSocket (on platforms that support them).  Socket
 type can be either `stream` or `dgram`; if omitted Git will