Add "language-option" to -list-features

Add "language-option" to -list-features

Following the addition of the --language optiton to all GDB/MI
commands,  I realized that there was no easy way for front-ends
to figure out whether this features is available or not. So I added
a "language-option" entry to -list-features.

gdb/ChangeLog:

        * mi/mi-main.c (mi_cmd_list_features): Add "language-options"
        to -list-features output.

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Miscellaneous Commands): Document the new
        "language-option" entry in the output of the "-list-features"
        command.