(DOCS-11776): add examples of WT cacheSizeGB default calculation

Author: nlarew

source/includes/extracts-wired-tiger.yaml

@@ -175,6 +175,12 @@ content: |
    - 50% of (RAM - 1 GB), or
 
    - 256 MB.
+
+   For example, on a system with a total of 4GB of RAM the WiredTiger
+   cache will use 1.5GB of RAM (``0.5 * (4 GB - 1 GB) = 1.5 GB``).
+   Conversely, a system with a total of 1.25 GB of RAM will allocate 256
+   MB to the WiredTiger cache because that is more than half of the
+   total RAM minus one gigabyte (``0.5 * (1.25 GB - 1 GB) = 128 MB < 256 MB``).
 ---
 ref: wt-filesystem-cache
 content: |