Reformat docs as markdown and correct spelling errors

Author: richardash1981

docs/modbus_read_file_record.md

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+# modbus_read_file_record
+
+## Name
+
+modbus_read_file_record - Read records from a file
+
+## Synopsis
+
+```c
+int modbus_read_file_record(modbus_t *'ctx', int 'addr', int 'sub_addr', int 'nb', uint16_t * 'dest');
+```
+
+## Description
+
+The *modbus_read_file_record()* function reads `nb` records from file
+number `addr`, starting from record position `sub_addr` in the file.
+The result of reading is stored in `dest` array as word values (16 bits). You
+must take care to allocate enough memory to store the results in `dest`
+- (at least `nb * sizeof(uint16_t)`).
+
+A file is an array of records, each of 16 bits.
+
+* A ModBus device may have up to 65535 files, addressed 1 to 65535 decimal.
+* Each file contains 10000 records, addressed 0000 to 9999 decimal.
+
+A maximum of 124 records (`nb`) may be retrieved in a single request.
+
+This function uses the ModBus function code 0x14 (Read File Record). Note that
+although the ModBus Specification allows for multiple non-contiguous reads in
+the same file to be made in a single request, this function only supports a
+single contiguous read request.
+
+
+## Return value
+
+The function shall return the number of records read (i.e. the value of `nb`) if successful.
+Otherwise it shall return -1 and set errno.
+
+
+## See also
+
+- [modbus_read_register](modbus_read_register.md)
+- [modbus_write_file_record](modbus_write_file_record.md)

docs/modbus_read_file_record.txt

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+modbus_write_file_record
+
+## Name
+
+modbus_write_file_record - Write records to a file
+
+## Synopsis
+
+```c
+int modbus_write_file_record(modbus_t *'ctx', int 'addr', int 'sub_addr', int 'nb', const uint16_t * 'src');
+```
+
+## Description
+
+The *modbus_write_file_record()* function writes the content of `nb`
+records from the `src` array to file number `addr`, starting from position
+`sub_addr` in the file.
+
+A file is an array of records, each of 16 bits.
+
+* A ModBus device may have up to 65535 files, addressed 1 to 65535 decimal.
+* Each file contains 10000 records, addressed 0000 to 9999 decimal.
+
+A maximum of 124 records (`nb`) may be written in a single request.
+
+This function uses the ModBus function code 0x15 (Write File Record). Note that
+although the ModBus Specification allows for multiple non-contiguous writes in
+the same file to be made in a single request, this function only supports a
+single contiguous write request.
+
+## Return value
+
+The function shall return the number of records written (i.e. the value of `nb`) if successful.
+Otherwise it shall return -1 and set errno.
+
+## See also
+
+- [modbus_write_register](modbus_write_register.md)
+- [modbus_read_file_record](modbus_read_file_record.md)