ExternalProject: Improve documentation

- Added clearer structure by grouping the options into logical sections.
- Expanded the details for many of the options.
- Added Examples section to show how to use the various commands.
- Specifically highlighted that the contents of SOURCE_DIR may be lost
  if a download method is also provided.
- Updated argument-matching regex to be more robust and account for the
  varying leading spaces before keywords in the docs.
- Updated tests to account for slightly changed error messages.

Tests/RunCMake/ExternalProject/NoOptions-stderr.txt

@@ -6,12 +6,12 @@
   is not an existing non-empty directory.  Please specify one of:
 
    \* SOURCE_DIR with an existing non-empty directory
+   \* DOWNLOAD_COMMAND
    \* URL
    \* GIT_REPOSITORY
+   \* SVN_REPOSITORY
    \* HG_REPOSITORY
    \* CVS_REPOSITORY and CVS_MODULE
-   \* SVN_REVISION
-   \* DOWNLOAD_COMMAND
 Call Stack \(most recent call first\):
   .*/Modules/ExternalProject.cmake:[0-9]+ \(_ep_add_download_command\)
   NoOptions.cmake:[0-9]+ \(ExternalProject_Add\)

Tests/RunCMake/ExternalProject/SourceEmpty-stderr.txt

@@ -6,12 +6,12 @@
   is not an existing non-empty directory.  Please specify one of:
 
    \* SOURCE_DIR with an existing non-empty directory
+   \* DOWNLOAD_COMMAND
    \* URL
    \* GIT_REPOSITORY
+   \* SVN_REPOSITORY
    \* HG_REPOSITORY
    \* CVS_REPOSITORY and CVS_MODULE
-   \* SVN_REVISION
-   \* DOWNLOAD_COMMAND
 Call Stack \(most recent call first\):
   .*/Modules/ExternalProject.cmake:[0-9]+ \(_ep_add_download_command\)
   SourceEmpty.cmake:[0-9]+ \(ExternalProject_Add\)

Tests/RunCMake/ExternalProject/SourceMissing-stderr.txt

@@ -6,12 +6,12 @@
   is not an existing non-empty directory.  Please specify one of:
 
    \* SOURCE_DIR with an existing non-empty directory
+   \* DOWNLOAD_COMMAND
    \* URL
    \* GIT_REPOSITORY
+   \* SVN_REPOSITORY
    \* HG_REPOSITORY
    \* CVS_REPOSITORY and CVS_MODULE
-   \* SVN_REVISION
-   \* DOWNLOAD_COMMAND
 Call Stack \(most recent call first\):
   .*/Modules/ExternalProject.cmake:[0-9]+ \(_ep_add_download_command\)
   SourceMissing.cmake:[0-9]+ \(ExternalProject_Add\)