doc: Correct various Texinfo warnings about incorrect use of macros. * install.txi: Change @strong{Note:} to @strong{Note}: to move colon outside of macro. * edit.m: Change "WARNING!" to "Warning:" for consistency with other instances. * wilcoxon_test.m: Change "Warning:" to "Caution:" to more accurately reflect level of concern. * strmatch.m: Change @strong{Note:} to @strong{Caution:} to stop spurious cross-reference creation. * datenum.m: Change "Warning:" to "Caution:" to more accurately reflect level of concern. * oct-parse.yy: Change @xref to an @pxref to suppress warning about incorrect usage.