[Patch V2] Build spec: Add description about auto detect thread number

[Yonghong Zhu <yonghong.zhu@intel.com>]

V2: add info that when value set to 1 means disable multiple thread.

Add description about enable auto detect thread number when
MAX_CONCURRENT_THREAD_NUMBER is not specified.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=775
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Kevin W Shaw <kevin.w.shaw@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
---
 5_meta-data_file_specifications/53_target_txt_file.md | 7 ++++---
 appendix_d_buildexe_command/d4_usage.md               | 3 +--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/5_meta-data_file_specifications/53_target_txt_file.md b/5_meta-data_file_specifications/53_target_txt_file.md
index ff8c2ec..8fe77eb 100644
--- a/5_meta-data_file_specifications/53_target_txt_file.md
+++ b/5_meta-data_file_specifications/53_target_txt_file.md
@@ -116,13 +116,14 @@ Specify the name of the `tools_def.txt` tool chain tag name to use. If not
 specified in this file and it is not specified using the `-t` option on the
 command-line, then the build will break.
 
 **_Integer_**
 
-The number of concurrent threads. The default, if not specified or set to zero,
-is 2 Recommend setting this value to one more than the number of computer cores
-or CPUs of the development workstation.
+The number of concurrent threads. If not specified or set to zero, tool
+automatically detect number of processor threads. Recommend setting this
+value to one less than the number of computer cores or CPUs of the development
+workstation. When value set to 1, means disable multithread build.
 
 **_BuildRulesFile_**
 
 Specify the file name to use for the build rules that are followed when
 generating Makefiles. If not specified, the file:
diff --git a/appendix_d_buildexe_command/d4_usage.md b/appendix_d_buildexe_command/d4_usage.md
index 57d2656..c9777ef 100644
--- a/appendix_d_buildexe_command/d4_usage.md
+++ b/appendix_d_buildexe_command/d4_usage.md
@@ -60,12 +60,11 @@ Options:
   -x SKUID, --sku-id=SKUID
                         Using this name of SKU ID to build the platform,
                         overriding SKUID_IDENTIFIER in DSC file.
   -n THREADNUMBER       Build the platform using multi-threaded compiler. The
                         value overrides target.txt's
-                        MAX_CONCURRENT_THREAD_NUMBER. Less than 2 will disable
-                        multi-thread builds.
+                        MAX_CONCURRENT_THREAD_NUMBER.
   -f FDFFILE, --fdf=FDFFILE
                         The name of the FDF file to use, which overrides the
                         setting in the DSC file.
   -r ROMIMAGE, --rom-image=ROMIMAGE
                         The name of FD to be generated. The name must be from
-- 
2.6.1.windows.1