swab.1: add swab(1) man page

docs/swab.1

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+.\" Copyright (c) 2024 DTB <trinity@trinity.moe>
+.\"
+.\" This work is licensed under CC BY-SA 4.0. To see a copy of this license,
+.\" visit <http://creativecommons.org/licenses/by-sa/4.0/>.
+
+.TH swab 1
+
+.SH NAME
+
+swab \(en swap bytes
+
+.SH SYNOPSIS
+
+swab
+.RB ( -f )
+.RB ( -w
+.R [
+.B word size
+.R ])
+
+.SH USAGE
+
+Swab swaps the latter and former halves of a block of bytes.
+
+.SH EXAMPLES
+
+The following sh(1p) line:
+
+.R printf 'hello world!\n' | swab
+
+Produces the following output:
+
+.R ehll oowlr!d
+
+.SH OPTIONS
+
+The
+.B -f
+option ignores system call interruptions.
+.PP
+The
+.B -w
+option configures the word size; that is, the size in bytes of the block size
+on which to operate. By default the word size is 2. The word size must be
+cleanly divisible by 2, otherwise the block of bytes being processed can't be
+halved.
+
+.SH DIAGNOSTICS
+
+If an error is encountered in input, output, or invocation, a diagnostic
+message will be written to standard error and swab will exit with the
+appropriate status from sysexits.h(3).
+
+.SH RATIONALE
+
+Swab was modeled after the
+.R conv=swab
+functionality specified in the POSIX dd utility but additionally allows the
+word size to be configured.
+.PP
+Swab is useful for fixing the endianness of binary files produced on other
+machines.
+
+.SH COPYRIGHT
+
+Copyright (c) 2024 DTB. License AGPLv3+: GNU AGPL version 3 or later
+<https://gnu.org/licenses/agpl.html>.
+
+.SH SEE ALSO
+
+dd(1p)