Resolve "Admin Area's "Account and limit" fields could be improved"

What does this MR do?

This MR adds clarifying information to the form found at Admin Area > Settings > General under the “Account and limit” section.

The following will be completed:

• Add units of measure to the Default projects limit field within the label
• Change the Session expiration label to “Session duration”
• Add a tooltip to all 4 fields that describe the field’s purpose
• Add Maximum push size to the section’s introduction

Screenshots

Before	After
	Screen_Recording_2020-01-27_at_7.56.10_AM

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #28872 (closed)