Improving description of MBED_OS_DIR config option

Add some text to make it clear that this config
option should be an absolute path.

Author: mark-edgeworth

docs/tools/CLI/cli-create.md

@@ -81,25 +81,27 @@ $ mbed new mbed-classic-program --mbedlib
 You can create plain (empty) programs, without either Mbed OS 5 or Mbed OS 2, by using the `--create-only` option.
 
 ### Managing multiple Mbed projects
-
-You can create multiple Mbed projects and use the same Mbed OS library directory for each of these projects with the following commands:
+By default there is a copy of mbed-os checked-out into each of your active mbed project directories.
+The configuration option ```MBED_OS_DIR``` allows you to create multiple mbed projects and use the same Mbed OS library directory for each of these projects.
+The ```MBED_OS_DIR``` configuration option should be set as an absolute path to a directory containing an implementation of mbed-os.
+The following commands illustrate this:
 
 ```
-$ cd <projects directory>
+$ cd /absolute/path/to/project/dir
 $ mbed import mbed-os
-$ mbed config -G MBED_OS_DIR <projects directory>/mbed-os
-[mbed] <projects directory>/mbed-os now set as global MBED_OS_DIR
+$ mbed config -G MBED_OS_DIR /absolute/path/to/project/dir/mbed-os
+[mbed] /absolute/path/to/project/dir/mbed-os now set as global MBED_OS_DIR
 $ mbed new project1
 [mbed] Creating new program "project1" (git)
 $ mbed new project2
 [mbed] Creating new program "project2" (git)
 ```
 
-Add your `main.cpp` file and other project files to the `project1` and `project2` directories. Then compile each project from the root `<projects directory>` with the following example commands:
+Add your `main.cpp` file and other project files to the `project1` and `project2` directories. Then compile each project from the root `/absolute/path/to/project/dir` with the following example commands:
 
 ```
 $ mbed compile -t ARM -m LPC1768 --source project1 --source mbed-os --build BUILD/project1
-$ mbed compile -t ARM -m K64F --source project2 --source mbed-os --build BUILD/project2  
+$ mbed compile -t ARM -m K64F --source project2 --source mbed-os --build BUILD/project2
 ```
 
 Find more details on the `--source` switch in the [build rules documentation](../reference/mbed-os-build-rules.html).