Document the database format and SQL code generation;

suggested by kristaps@ and espie@.

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+.\"	$Id: mandoc.db.5,v 1.1 2014/04/15 20:18:26 schwarze Exp $
+.\"
+.\" Copyright (c) 2014 Ingo Schwarze <schwarze@openbsd.org>
+.\"
+.\" Permission to use, copy, modify, and distribute this software for any
+.\" purpose with or without fee is hereby granted, provided that the above
+.\" copyright notice and this permission notice appear in all copies.
+.\"
+.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+.\"
+.Dd $Mdocdate: April 15 2014 $
+.Dt MANDOC.DB 5
+.Os
+.Sh NAME
+.Nm mandoc.db
+.Nd manual page database
+.Sh DESCRIPTION
+The
+.Nm
+SQLite3 file format is used to store information about installed manual
+pages to facilitate semantic searching for manuals.
+Each manual page tree contains its own
+.Nm
+file; see
+.Sx FILES
+for examples.
+.Pp
+Such database files are generated by
+.Xr makewhatis 8
+and used by
+.Xr apropos 1
+and
+.Xr whatis 1 .
+.Pp
+One line in the following tables describes:
+.Bl -tag -width Ds
+.It Sy mpages
+One physical manual page file, no matter how many times and under which
+names it may appear in the file system.
+.It Sy mlinks
+One entry in the file system, no matter which content it points to.
+.It Sy names
+One manual page name, no matter whether it appears in a page header,
+in a NAME or SYNOPSIS section, or as a file name.
+.It Sy keys
+One chunk of text from some macro invocation.
+.El
+.Pp
+Each record in the latter three tables uses its
+.Va pageid
+column to point to a record in the
+.Sy mpages
+table.
+.Pp
+The other columns are as follows; unless stated otherwise, they are
+of type
+.Vt TEXT .
+.Bl -tag -width mpages.desc
+.It Sy mpages.desc
+The description line
+.Pq Sq \&Nd
+of the page.
+.It Sy mpages.form
+The
+.Vt INTEGER
+1 if the page is unformatted, i.e. in
+.Xr mdoc 7
+or
+.Xr man 7
+format, and 2 if it is formatted, i.e. a
+.Sq cat
+page.
+.It Sy mlinks.sec
+The manual section as found in the subdirectory name.
+.It Sy mlinks.arch
+The manual architecture as found in the subdirectory name, or
+.Qq any .
+.It Sy mlinks.name
+The manual name as found in the file name.
+.It Sy names.bits
+An
+.Vt INTEGER
+bit mask telling whether the name came from a header line, from the
+NAME or SYNOPSIS section, or from a file name.
+Bits are defined in
+.In mansearch.h .
+.It Sy names.name
+The name itself.
+.It Sy keys.bits
+An
+.Vt INTEGER
+bit mask telling which semantic contexts the key was found in;
+defined in
+.In mansearch.h ,
+documented in
+.Xr apropos 1 .
+.It Sy keys.key
+The string found in those contexts.
+.El
+.Sh FILES
+.Bl -tag -width /usr/share/mandoc.db -compact
+.It Pa /usr/share/mandoc.db
+The manual page database for the base system.
+.It Pa /usr/X11R6/mandoc.db
+The same for the
+.Xr X 7
+Window System.
+.It Pa /usr/local/mandoc.db
+The same for
+.Xr packages 7 .
+.El
+.Sh SEE ALSO
+.Xr apropos 1 ,
+.Xr man 1 ,
+.Xr sqlite3 1 ,
+.Xr whatis 1 ,
+.Xr mansearch 3 ,
+.Xr makewhatis 8
+.Sh HISTORY
+A manual page database
+.Pa /usr/lib/whatis
+first appeared in
+.Bx 2 .
+The present format first appeared in
+.Ox 5.6 .
+.Sh AUTHORS
+.An -nosplit
+The original version of
+.Xr makewhatis 8
+was written by
+.An Bill Joy
+in 1979.
+An SQLite3 version was first implemented by
+.An Kristaps Dzonsons Aq Mt kristaps@bsd.lv
+in 2012.
+The present database format was designed by
+.An Ingo Schwarze Aq Mt schwarze@openbsd.org
+in 2014.