doc: improve fs.StatFs property descriptions and add examples

Add missing information to the fs.StatFs class documentation:

- Clarify that bsize and frsize values are in bytes
- Add ESM and CJS examples showing how to calculate disk space
- Explain the relationship between block counts and bsize
- Clarify the difference between bavail (unprivileged) and bfree (total)
- Document that type is a filesystem magic number with common values
- Add cross-references between related properties

Fixes: #50749

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: marcelsafin

doc/api/fs.md

@@ -7926,6 +7926,32 @@ StatFs {
 }
 ```
 
+To calculate disk space from a `StatFs` object, multiply the block count
+properties by the block size (`statfs.bsize`) to convert them to bytes.
+Pass `{ bigint: true }` to avoid loss of precision on large file systems:
+
+```mjs
+import { statfs } from 'node:fs/promises';
+
+const stats = await statfs('/', { bigint: true });
+
+console.log(`Total: ${stats.blocks * stats.bsize} bytes`);
+console.log(`Free:  ${stats.bfree * stats.bsize} bytes`);
+console.log(`Available: ${stats.bavail * stats.bsize} bytes`);
+```
+
+```cjs
+const { statfs } = require('node:fs');
+
+statfs('/', { bigint: true }, (err, stats) => {
+  if (err) throw err;
+
+  console.log(`Total: ${stats.blocks * stats.bsize} bytes`);
+  console.log(`Free:  ${stats.bfree * stats.bsize} bytes`);
+  console.log(`Available: ${stats.bavail * stats.bsize} bytes`);
+});
+```
+
 #### `statfs.bavail`
 
 <!-- YAML
@@ -7936,7 +7962,9 @@ added:
 
 * Type: {number|bigint}
 
-Free blocks available to unprivileged users.
+Free blocks available to unprivileged users. Unlike [`statfs.bfree`][],
+this value excludes blocks reserved for privileged processes. Use
+`statfs.bavail * statfs.bsize` to get the available space in bytes.
 
 #### `statfs.bfree`
 
@@ -7948,7 +7976,10 @@ added:
 
 * Type: {number|bigint}
 
-Free blocks in file system.
+Free blocks in file system, including reserved blocks that are not
+available to unprivileged users. See [`statfs.bavail`][] for the
+unreserved count. Use `statfs.bfree * statfs.bsize` to get the free
+space in bytes.
 
 #### `statfs.blocks`
 
@@ -7960,7 +7991,8 @@ added:
 
 * Type: {number|bigint}
 
-Total data blocks in file system.
+Total data blocks in file system. Use `statfs.blocks * statfs.bsize`
+to get the total size in bytes.
 
 #### `statfs.bsize`
 
@@ -7972,7 +8004,9 @@ added:
 
 * Type: {number|bigint}
 
-Optimal transfer block size.
+Optimal transfer block size, in bytes. This value can be used to convert
+block count properties (such as [`statfs.blocks`][], [`statfs.bfree`][],
+and [`statfs.bavail`][]) to bytes.
 
 #### `statfs.frsize`
 
@@ -7982,7 +8016,7 @@ added: REPLACEME
 
 * Type: {number|bigint}
 
-Fundamental file system block size.
+Fundamental file system block size, in bytes.
 
 #### `statfs.ffree`
 
@@ -7994,7 +8028,7 @@ added:
 
 * Type: {number|bigint}
 
-Free file nodes in file system.
+Free file nodes (inodes) in file system.
 
 #### `statfs.files`
 
@@ -8006,7 +8040,7 @@ added:
 
 * Type: {number|bigint}
 
-Total file nodes in file system.
+Total file nodes (inodes) in file system.
 
 #### `statfs.type`
 
@@ -8018,7 +8052,9 @@ added:
 
 * Type: {number|bigint}
 
-Type of file system.
+Type of file system. On Linux this is a filesystem magic number (for
+example, `0xEF53` for ext2/ext3/ext4). The value is platform-dependent
+and may be `0` on systems where libuv does not report it.
 
 ### Class: `fs.Utf8Stream`
 
@@ -9052,6 +9088,10 @@ the file contents.
 [`fs.rmdir()`]: #fsrmdirpath-options-callback
 [`fs.stat()`]: #fsstatpath-options-callback
 [`fs.statfs()`]: #fsstatfspath-options-callback
+[`statfs.bavail`]: #statfsbavail
+[`statfs.bfree`]: #statfsbfree
+[`statfs.blocks`]: #statfsblocks
+[`statfs.bsize`]: #statfsbsize
 [`fs.symlink()`]: #fssymlinktarget-path-type-callback
 [`fs.utimes()`]: #fsutimespath-atime-mtime-callback
 [`fs.watch()`]: #fswatchfilename-options-listener