Revert "ITS#8977 - Remove documentation for idlexp"

This reverts commit a40f6bff.

doc/man/man5/slapd-mdb.5

@@ -17,6 +17,18 @@ primary database backend.
 The \fBmdb\fP backend uses a hierarchical database layout which
 supports subtree renames.
 .SH CONFIGURATION
+These
+.B slapd.conf
+options apply to the \fBmdb\fP backend.
+That is, they must follow a "backend mdb" line and
+come before any subsequent "backend" or "database" lines.
+.TP
+.BI idlexp \ <exp>
+Specify a power of 2 for the maximum size of an index slot.
+The default is 16, yielding a maximum slot size of 2^16 or 65536.
+Once set, this option applies to every \fBmdb\fP database instance.
+The specified value must be in the range of 16-31.
+.LP
 
 These
 .B slapd.conf