[docs] Updating documentation for compdb

Update gn gen docs to include the --export-compile-commands flag

Change-Id: Id3afbc6270ef593fb12ecbb22ca78dd516dc5150
Reviewed-on: https://gn-review.googlesource.com/2500
Commit-Queue: Brett Wilson <brettw@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>

docs/reference.md

diff --git a/docs/reference.md b/docs/reference.md
index 0998620..98a3fdd 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -745,6 +745,17 @@
   --json-ide-script-args=<argument>
       Optional second argument that will passed to executed script.
 ```
+
+#### **Compilation Database**
+
+```
+  --export-compile-commands
+      Produces a compile_commands.json file in the root of the build directory
+      containing an array of “command objects”, where each command object
+      specifies one way a translation unit is compiled in the project. This is
+      used for various Clang-based tooling, allowing for the replay of individual
+      compilations independent of the build system.
+```
 ### <a name="help"></a>**gn help <anything>**
 
 ```

tools/gn/command_gen.cc

diff --git a/tools/gn/command_gen.cc b/tools/gn/command_gen.cc
index d959fe4..82046d2 100644
--- a/tools/gn/command_gen.cc
+++ b/tools/gn/command_gen.cc
@@ -412,6 +412,15 @@
 
   --json-ide-script-args=<argument>
       Optional second argument that will passed to executed script.
+
+Compilation Database
+
+  --export-compile-commands
+      Produces a compile_commands.json file in the root of the build directory
+      containing an array of “command objects”, where each command object
+      specifies one way a translation unit is compiled in the project. This is
+      used for various Clang-based tooling, allowing for the replay of individual
+      compilations independent of the build system.
 )";
 
 int RunGen(const std::vector<std::string>& args) {