docs: add @return, @details, and example to type_convert() documentation

[LeonidasZhak]

What

Adds missing documentation to type_convert():

1. @return tag: Documents that the function returns a data frame with character columns converted to guessed types, and that unparsable values become NA with a warning.

2. @details section: Explains that column types are guessed using guess_parser(), and notes that the numeric parser recognizes some alphanumeric strings as valid numbers (e.g., "100e5", "100L", "100S"). Suggests using explicit cols() specifications to avoid unexpected parsing.

3. Example: Shows parsing failure with explicit col_types = cols(x = col_double()).

Why

Partially addresses #1562. While the underlying Qi parser behavior (treating F/L/S as floating-point suffixes) requires a C++ code fix, this PR documents the current behavior so users can understand and work around it.

Validation

• tools::checkRd("man/type_convert.Rd") — clean
• Example runs correctly:

  df <- data.frame(x = c("100", "200", "abc"), stringsAsFactors = FALSE)
  type_convert(df, col_types = cols(x = col_double()))
  # Warning: [2, 1]: expected a double, but got 'abc'
  #     x
  # 1 100
  # 2 200
  # 3  NA

Files changed

• R/type_convert.R (roxygen source)
• man/type_convert.Rd (generated)

[LeonidasZhak]

Withdrawing this small automated PR while I consolidate an oversized batch of contributions and reduce maintainer review burden. Sorry for the noise, and thank you for maintaining the project.