Adjust spawning_biomass documentation feeedback

[Schiano-NOAA]

Major Changes

• Feedback from [Documentation Review] plot_spawning_biomass #197 is addressed in this PR
• The documentation of all arguments has been clarified and format has been improved
• The examples in both spawning_biomass and plot_biomass have been updated so no errors or warnings are produced to reduce confusion of users
• Adjusted conditions when some error messages are produced

[github-actions]

New version checklist

• Package version in DESCRIPTION has been updated
• Release notes have been drafted/published
• Cheatsheet content has been updated (if applicable)
• Cheatsheet version has been updated

[github-actions]

Code Metrics Report

Coverage	Code to Test Ratio	Test Execution Time
70.9%	1:0.1	4m44s

Code coverage of files in pull request scope (85.5%)

Files	Coverage
R/plot_biomass.R	90.1%
R/plot_spawning_biomass.R	78.9%
R/utils_plot.R	86.1%

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Reported by octocov