[Website]: Add Information about root reservation preventing frozen nodes from starting

### Type of issue

Missing information

### What documentation page is affected

https://www.elastic.co/docs/reference/elasticsearch/configuration-reference/node-settings

### What happened?

Hi team,
Filesystems may sometimes be created with a root reservation. When starting a frozen node this may lead to a failure of one of the bootstrap checks for free space :

`Not enough free space [4818024595456] for cache file of size [4966425034752] in path [/]","error.stack_trace":"java.io.UncheckedIOException: java.io.IOException: Not enough free space [4818024595456] for cache file of size [4966425034752] in path [/]`

The issue may not be clear at first for customers who just prepped a new machine and have a blank filesystem but get an error from ES.
To help with troubleshooting we could explain the importance of checking there is no root reservation.

It will manifest like this when using `df -h` : `SIZE : 4.7T | USED 20K | AVAIL 4.4T | USE 1%`. ES checks total size, but with the reservation, there is only 4.4 TB available even though this is an empty path and use reports 1%.

### Additional info

Linked to https://github.com/elastic/enhancements/issues/26793