docs(commons): improve / rephrase documentation

[J-Loudet]

• zenoh-flow-commons/src/configuration.rs: improve documentation of Configuration structure.
• zenoh-flow-commons/src/deserialize.rs: rephrased errors to use affirmative "tense".
• zenoh-flow-commons/src/utils.rs:
  • internal documentation of deserializer function (to obtain the function that would deserialise a structure based on the Path of the file),
  • internal documentation of error cases when attempting to parse a structure from a file.
• zenoh-flow-commons/src/vars.rs: improve documentation and explain error case when parsing vars from a String.