Update docs about disk usage metrics (#3876)

docs/en/observability/monitor-infra/host-metrics.asciidoc

@@ -34,7 +34,7 @@ Learn about key host metrics displayed in the {infrastructure-app}:
 |===
 | Metric                      | Description
 
-| **CPU Usage (%)**          | Percentage of CPU time spent in states other than Idle and IOWait, normalized by the number of CPU cores. This includes both time spent on user space and kernel space. 
+| **CPU Usage (%)**          | Percentage of CPU time spent in states other than Idle and IOWait, normalized by the number of CPU cores. This includes both time spent on user space and kernel space.
 
 100% means all CPUs of the host are busy.
 
@@ -70,27 +70,27 @@ Learn about key host metrics displayed in the {infrastructure-app}:
 
 | **Load (1m)**              | 1 minute load average.
 
-Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete). 
+Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete).
 
 **Field Calculation:** average(system.load.1)
 
 | **Load (5m)**              | 5 minute load average.
 
-Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete). 
+Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete).
 
 **Field Calculation:** `average(system.load.5)`
 
 | **Load (15m)**             | 15 minute load average.
 
-Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete). 
+Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete).
 
 **Field Calculation:** `average(system.load.15)`
 
-| **Normalized Load**        | 1 minute load average normalized by the number of CPU cores. 
+| **Normalized Load**        | 1 minute load average normalized by the number of CPU cores.
 
-Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete). 
+Load average gives an indication of the number of threads that are runnable (either busy running on CPU, waiting to run, or waiting for a blocking IO operation to complete).
 
-100% means the 1 minute load average is equal to the number of CPU cores of the host. 
+100% means the 1 minute load average is equal to the number of CPU cores of the host.
 
 Taking the example of a 32 CPU cores host, if the 1 minute load average is 32, the value reported here is 100%. If the 1 minute load average is 48, the value reported here is 150%.
 
@@ -122,7 +122,7 @@ Taking the example of a 32 CPU cores host, if the 1 minute load average is 32, t
 
 **Field Calculation:** `avg(system.memory.total)`
 
-| **Memory Usage (%)**       | Percentage of main memory usage excluding page cache. 
+| **Memory Usage (%)**       | Percentage of main memory usage excluding page cache.
 
 This includes resident memory for all processes plus memory used by the kernel structures and code apart from the page cache.
 
@@ -191,9 +191,9 @@ A high level indicates a situation of memory saturation for the host. For exampl
 
 **Field Calculation:**  `1-average(system.filesystem.used.pct)`
 
-| **Disk Usage - Used (%)**         | Percentage of disk space used.
+| **Disk Usage - Max (%)**         | Percentage of disk space used. A high percentage indicates that a partition on a disk is running out of space.
 
-**Field Calculation:**  `average(system.filesystem.used.pct)`
+**Field Calculation:**  `max(system.filesystem.used.pct)`
 
 | **Disk Write IOPS**              | Average count of write operations from the device per second.
 
@@ -204,4 +204,3 @@ A high level indicates a situation of memory saturation for the host. For exampl
 **Field Calculation:**  `counter_rate(max(system.diskio.write.bytes), kql='system.diskio.write.bytes: *')`
 
 |===
-