tools: bpftool: document restriction on '.' in names to pin in bpffs

Names used to pin eBPF programs and maps under the eBPF virtual file
system cannot contain a dot character, which is reserved for future
extensions of this file system.

Document this in bpftool man pages to avoid users getting confused if
pinning fails because of a dot.

Signed-off-by: Quentin Monnet <[e-mail]>
Reviewed-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

tools/bpf/bpftool/Documentation/bpftool-map.rst

···
 	**bpftool map pin**     *MAP*  *FILE*
 		  Pin map *MAP* as *FILE*.
 
-		  Note: *FILE* must be located in *bpffs* mount.
+		  Note: *FILE* must be located in *bpffs* mount. It must not
+		  contain a dot character ('.'), which is reserved for future
+		  extensions of *bpffs*.
 
 	**bpftool** **map event_pipe** *MAP* [**cpu** *N* **index** *M*]
 		  Read events from a BPF_MAP_TYPE_PERF_EVENT_ARRAY map.

tools/bpf/bpftool/Documentation/bpftool-prog.rst

···
 	**bpftool prog pin** *PROG* *FILE*
 		  Pin program *PROG* as *FILE*.
 
-		  Note: *FILE* must be located in *bpffs* mount.
+		  Note: *FILE* must be located in *bpffs* mount. It must not
+		  contain a dot character ('.'), which is reserved for future
+		  extensions of *bpffs*.
 
 	**bpftool prog load** *OBJ* *FILE* [**type** *TYPE*] [**map** {**idx** *IDX* | **name** *NAME*} *MAP*] [**dev** *NAME*]
 		  Load bpf program from binary *OBJ* and pin as *FILE*.
···
 		  If **dev** *NAME* is specified program will be loaded onto
 		  given networking device (offload).
 
-		  Note: *FILE* must be located in *bpffs* mount.
+		  Note: *FILE* must be located in *bpffs* mount. It must not
+		  contain a dot character ('.'), which is reserved for future
+		  extensions of *bpffs*.
 
         **bpftool prog attach** *PROG* *ATTACH_TYPE* *MAP*
                   Attach bpf program *PROG* (with type specified by *ATTACH_TYPE*)