Make header file compatible with doxygen

[sebromero]

Without the double asterisk and the @file declaration, doxygen won't render any documentation for global functions. This PR fixes this.

[github-actions]

Memory usage change @ 64369cb

Board	flash	%	RAM for global variables	%
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A
arduino:renesas_portenta:portenta_c33	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/SD_Card_Example flash	%	examples/SD_Card_Example RAM for global variables	%	examples/USB_Hotplug_Example flash	%	examples/USB_Hotplug_Example RAM for global variables	%	examples/USB_No_Hotplug_Example flash	%	examples/USB_No_Hotplug_Example RAM for global variables	%	examples/tests/Arduino_POSIXStorage_Test flash	%	examples/tests/Arduino_POSIXStorage_Test RAM for global variables	%
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
arduino:renesas_portenta:portenta_c33	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0

Click for full report CSV

Board,examples/SD_Card_Example<br>flash,%,examples/SD_Card_Example<br>RAM for global variables,%,examples/USB_Hotplug_Example<br>flash,%,examples/USB_Hotplug_Example<br>RAM for global variables,%,examples/USB_No_Hotplug_Example<br>flash,%,examples/USB_No_Hotplug_Example<br>RAM for global variables,%,examples/tests/Arduino_POSIXStorage_Test<br>flash,%,examples/tests/Arduino_POSIXStorage_Test<br>RAM for global variables,%
arduino:mbed_portenta:envie_m7,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A
arduino:renesas_portenta:portenta_c33,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0

[github-actions]

Memory usage change @ e1f858a

Board	flash	%	RAM for global variables	%
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A
arduino:renesas_portenta:portenta_c33	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/SD_Card_Example flash	%	examples/SD_Card_Example RAM for global variables	%	examples/USB_Hotplug_Example flash	%	examples/USB_Hotplug_Example RAM for global variables	%	examples/USB_No_Hotplug_Example flash	%	examples/USB_No_Hotplug_Example RAM for global variables	%	examples/tests/Arduino_POSIXStorage_Test flash	%	examples/tests/Arduino_POSIXStorage_Test RAM for global variables	%
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
arduino:renesas_portenta:portenta_c33	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0

Click for full report CSV

Board,examples/SD_Card_Example<br>flash,%,examples/SD_Card_Example<br>RAM for global variables,%,examples/USB_Hotplug_Example<br>flash,%,examples/USB_Hotplug_Example<br>RAM for global variables,%,examples/USB_No_Hotplug_Example<br>flash,%,examples/USB_No_Hotplug_Example<br>RAM for global variables,%,examples/tests/Arduino_POSIXStorage_Test<br>flash,%,examples/tests/Arduino_POSIXStorage_Test<br>RAM for global variables,%
arduino:mbed_portenta:envie_m7,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A
arduino:renesas_portenta:portenta_c33,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0