Add descriptions of the nm(1) keywords and their meanings.

PR:             docs/17269
Submitted by:   Oscar Bonilla <obonilla@fisicc-ufm.edu>

usr.bin/nm/nm.1

@@ -114,6 +114,32 @@ warn if next symbol is referenced
 .Pp
 If the symbol is local (non-external) the type letter is in lower case.
 The output is sorted alphabetically.
+.Sh KEYWORDS
+The following is a complete list of keywords and their meaning.
+The keyword is uppercase for external, lowercase for internal symbols.
+Only uppercase keywords are shown here.
+.Pp
+.Bl -tag -width sigignore -compact
+.It A
+Absolute Address
+.It B
+BSS Segment
+.It C
+Common Reference
+.It D
+Data Segment
+.It F
+Filename (external only)
+.It w
+Warning refs (internal only)
+.It I
+Alias definition
+.It T
+Text Segment
+.It U
+Undefined
+.It ?
+None of the above
 .Sh SEE ALSO
 .Xr ar 1 ,
 .Xr a.out 5 ,