tools: bpftool: update documentation for --json and --pretty usage

Update the documentation to provide help about JSON output generation,
and add an example in bpftool-prog manual page.

Also reintroduce an example that was left aside when the tool was moved
from GitHub to the kernel sources, in order to show how to mount the
bpffs file system (to pin programs) inside the bpftool-prog manual page.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/bpf/bpftool/Documentation/bpftool.rst

@@ -10,7 +10,7 @@ tool for inspection and simple manipulation of eBPF programs and maps
 SYNOPSIS
 ========
 
-	**bpftool** *OBJECT* { *COMMAND* | **help** }
+	**bpftool** [*OPTIONS*] *OBJECT* { *COMMAND* | **help** }
 
 	**bpftool** **batch file** *FILE*
 
@@ -18,6 +18,9 @@ SYNOPSIS
 
 	*OBJECT* := { **map** | **program** }
 
+	*OPTIONS* := { { **-V** | **--version** } | { **-h** | **--help** }
+	| { **-j** | **--json** } [{ **-p** | **--pretty** }] }
+
 	*MAP-COMMANDS* :=
 	{ **show** | **dump** | **update** | **lookup** | **getnext** | **delete**
 	| **pin** | **help** }
@@ -41,6 +44,13 @@ OPTIONS
 	-v, --version
 		  Print version number (similar to **bpftool version**).
 
+	-j, --json
+		  Generate JSON output. For commands that cannot produce JSON, this
+		  option has no effect.
+
+	-p, --pretty
+		  Generate human-readable JSON output. Implies **-j**.
+
 SEE ALSO
 ========
 	**bpftool-map**\ (8), **bpftool-prog**\ (8)