Add client_max_body_size to PHP execution limits, and mention atom.conf PHP pool on this page

**Documentation version**

* 2.5; 2.6

**Relevant documentation pages**

* https://www.accesstomemory.org/docs/latest/admin-manual/installation/execution-limits/
* https://www.accesstomemory.org/docs/latest/admin-manual/installation/linux/ubuntu-xenial/
* https://www.accesstomemory.org/docs/latest/admin-manual/maintenance/troubleshooting/#why-can-t-i-upload-large-digital-objects

**Relevant forum thread**

* 2020-02-12: https://groups.google.com/d/msg/ica-atom-users/fkth3RTdPRU/iUEXHwmmAwAJ

**Issue description**

A user reported issues trying to upload a 72MB PDF. The issue was solved by adjusting the global php.ini execution limits, as well as adjusting the limits set in the atom.conf PHP pool (created during installation). However, to get it to work and resolve `413 Request Entity Too Large` error, the user also had to adjust the atom.conf value for `client_max_body_size`.

**Recommended solution**

1) Determine if this parameter is available in the global php.ini file. If yes, add mention of it to the Execution limits documentation page

2) Remind users on the Execution limits page that limits are also set during installation in the atom.conf file, and to adjust those as well during tuning and configuration.

3) Update the "Why can't I upload large digital objects" section of the Troubleshooting page with further information, including:

* pointing to the atom.conf file
* mentioning the `413 Request Entity Tool Large` error and pointing to the `client_max_body_size` Nginx config variable as a possible solution
* General review and updating based on FAQ forum posts on this topic

Start with the Execution limits and Troubleshooting pages listed in the issue, then review the Ubuntu installation page for the atom.conf PHP pool reference. Check the related forum thread for the upload-limit guidance. Done means the documentation covers PHP limits, atom.conf, client_max_body_size, and the 413 Request Entity Too Large error without leaving the relevant setup steps unclear.

Written by the indexing model from the issue text.