DOCSP-34246 -- Add note re: required exec privileges for /var directory (#29)

Add note re: required exec privileges for /var directory.

[JIRA](https://jira.mongodb.org/browse/DOCSP-34246)

[STAGING](https://preview-mongodbjvincentmongodb.gatsbyjs.io/ops-manager/DOCSP-34246/core/requirements/#ulimits-requirements-for-application)

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NOTE: this PR is linked to: 10gen/mms-docs#5330

Author: jvincent-mongodb

source/core/requirements.txt

@@ -447,10 +447,17 @@ Intel/AMD (``x86_64``) Hardware Architectures
 
    As of |onprem| Server 4.0.11, Windows architectures require
    the `Visual C++ Redistributable Packages for Visual Studio 2013
-   <https://www.microsoft.com/en-us/download/details.aspx?id=40784>`__.
+   <https://www.microsoft.com/en-us/download/details.aspx?id=40784>`__. 
 
 .. _onprem-config-ulimits:
 
+Exec Options for `/var` Mount
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+|onprem| requires the default ``exec`` option defined in 
+``/etc/fstab`` on the underlying host machine in order to run the required 
+binaries stored in the volume backing the ``/var`` directory.
+
 Ulimits Requirements for |application|
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~